What is LVM:-
LVM stands for Logical Volume
Manager and is a way of grouping disk drives and/or partition in a way
where instead of dealing with hard and fast physical partitions the data
is managed in a virtual basis where the virtual partitions can be
resized.
By using LVM, all
physical disk or partitions, no matter what size, how scattered they
are, cab be abstracted and viewed as a single storage source.
LVM is a method of grouping of multiple disks or partitions to create logical representation of one disk to os.
LVM
adds a logical layer between the physical disk (this can be a real
disk, RAID array, or something else) and the filesystem. This logical
layer (or a volume) then allows you to dynamically resize the logical
volume (if supported by the filesystem), transparently move the
filesystems to a different physical location, create read/write
snapshots of a filesystem and spread your data over multiple physical
locations.
Note:- There is one limitation that a LVM cannot be used for the /boot.
LVM organization:-
LVM is structured into three elements:
a) Volumes:- Physical and logical volumes & volume groups.
b) Extents:- Physical and logical extents.
c) Device-mapper:- Device mapper is modular linux kernel driver. It provides generic framework to map
one block device to other.
Device-mapper:-
The uapplication interface to the Device Mapper is the ioctl system call
The user interface is the dmsetup command.
The kernel space consists of ioctl interface, device mapper. Ioctl are interface that is used for
communication between the kernel space and user space. Device Mapper is a generic de vice
mapping facility that is used to map lower level hard disk with Logical Volume Management or
LVM.
Role of device-mapper:-
The role of device mapper is to maintain a table which contains complete information of all the
devices on which LVM is formed.
It stores all this information in a structure known as dm-table.
As the logical volume is created the device mapper maps and store the device information
in its table known as dm table. It is linear table which store all the information such as
begin, length and major and minor number which is required to indentify the device.
It contain another structure dm-target
which hold the logical begin offset and the length of a particular
device which are used to map the data from the user space to the
hardware level.
LVM Anatomy:-
Physical Volume:- A
physical volume (PV) is another name for a regular physical disk or
partition that is used or will be used by LVM. To use device as LVM
logical volume it must be initialized as PV,
initializing a block device as a physical volume places a label near the start of the device. By
default, the LVM label is placed in the second 512-byte sector. You can
overwrite this default by placing the label on any of the first 4
sectors. This allows LVM volumes to co-exist with other users of these
sectors, if necessary.
The LVM label identifies the device as
an LVM physical volume. It contains a random unique identifier (the
UUID) for the physical volume. It also stores the size of the block
device in bytes, and it records where the LVM metadata will be stored on
the device.
The LVM metadata
contains the configuration details of the LVM volume groups on your
system. By default, an identical copy of the metadata is maintained in
every metadata area in every physical volume within the volume group.
LVM metadata is small and stored as ASCII.
Currently
LVM allows you to store 0, 1 or 2 identical copies of its metadata on
each physical volume. The default is 1 copy. Once you configure the
number of metadata copies on the physical volume, you cannot change that
number at a later time. The first copy is stored at the start of the
device, shortly after the label. If there is a second copy, it is placed
at the end of the device. If you accidentally overwrite the area at the
beginning of your disk by writing to a different disk than you intend, a
second copy of the metadata at the end of the device will allow you to
recover the metadata.
It iscreated by pvcreate command.
Volume group:- Volume group is aggregation of Physical volumes. Volume
group is created when atleast one PV is added to it. The space from all
PV in VG is summed up to from large pool of storage, which is then used
to build one or more logical volume(LV), the disk space available for
allocation is divided into units of a fixed-size called extents. An
extent is the smallest unit of space that can be allocated, Within a
physical volume, extents are referred to as physical extents.
The
Volume Group is the highest level abstraction used within the LVM. It
gathers together a collection of Logical Volumes and Physical Volumes
into one administrative unit.
Physical Extent:-
When a volume group is created, the physical volume added to it is
divided into chunks of data called physical extents (Pes). An extent is
the smallest unit of space that can be allocated, Within a physical
volume, extents are referred to as physical extents.
Note: -
The
physical extent size (PE size) of a LVM volume group (VG) is fixed upon
the creation of VG. In Linux command line, the -s option switch of vgcreate
command is to explicitly set the physical extent size (PE size) on
physical volumes (PV) of the volume group (VG). PE size is defaulted to
4MB if it’s not set explicitly. However, once this value has been set,
it’s not possible to change a PE size without recreating the volume
group which would involve backing up and restoring data on any logical
volumes!
What’s the maximum size of a logical volume in Linux LVM?
Why there is a limitation on maximum size of logical volume?
In order to limit the Linux kernel memory usage, there is a limit of
65,536 physical extents (PE) per logical volume (LV). Hence, the LVM PE size will directly determine the maximum size of a logical volume (LV)!
For example, 4MB PE size (the default PE size) will limit single
logical volume (LV) to 256GB, 16MB PE size will limit single LV to grow
beyond 1TB, etc.
Beside the PE size, the maximum size of single LV is also limited by CPU architecture and Linux kernel version:
Linux kernel version 2.4.x limit the maximum LV size to 2TB.
Some
older Linux kernel prior to 2.4.x, the maximum LV size is limited to
1TB (caused by the integer signedness problems in the block layer).
The combination of 32-bit CPU and Linux kernel version 2.6.x, the limit of logical volume size is maximized at 16TB.
- For Linux kernel 2.6.x running on 64-bit CPU, the maximum LV size is 8EB (extremely terrible big storage for this time being!)
Logical Volume:- The Logical Volume is logical partition created from disk space assigned to the Volume Group. There are three types of LVM logical volumes: linear volumes, striped volumes, and mirrored volumes.
LV can be increased or decreased in size and can use space taken from several PV inside VG.
Logical Extent:- Each
logical volume is split into chunks of disk space, known as logical
extents. The extent size is the same for all logical volumes in the
volume group. LE points to the PE.
The PE and LE size s are usually kept same within VG. LE can be however smaller or larger than PE. The default LE size is 4Mb.
Physical Volume Administration:-
a) Initializing Physical Volumes
Use the pvcreate command to initialize a block device to be used as a physical volume. Initialization is analogous to formatting a file system.
The following command initializes /dev/sdd1, /dev/sde1, and /dev/sdf1 for use as LVM physical volumes.
pvcreate /dev/sdd1 /dev/sde1 /dev/sdf1
The following command initializes /dev/sdd1, /dev/sde1, and /dev/sdf1 and creates two metadata copies for use as LVM physical volumes
pvcreate –metadatacopies 2 /dev/sdd1 /dev/sde1 /dev/sdf1
b)Displaying Physical Volumes :-
There are three commands you can use to display properties of LVM physical volumes: pvs, pvdisplay, and pvscan.
The pvs command provides physical volume information in a configurable form, displaying one line per physical volume. The pvs command provides a great deal of format control, and is useful for scripting. For information on using the pvs command to customize your output.
The pvdisplay
command provides a verbose multi-line output for each physical volume.
It displays physical properties (size, extents, volume group, etc.) in a
fixed format.
The following example shows the output of the pvdisplay command for a single physical volume.
# pvdisplay
--- Physical volume ---
PV Name /dev/sdc1
VG Name new_vg
PV Size 17.14 GB / not usable 3.40 MB
Allocatable yes
PE Size (KByte) 4096
Total PE 4388
Free PE 4375
Allocated PE 13
PV UUID Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe
The pvscan command scans all supported LVM block devices in the system for physical volumes.
The following command shows all physical devices found:
# pvscan
PV /dev/sdb2 VG vg0 lvm2 [964.00 MB / 0 free]
PV /dev/sdc1 VG vg0 lvm2 [964.00 MB / 428.00 MB free]
PV /dev/sdc2 lvm2 [964.84 MB]
Total: 3 [2.83 GB] / in use: 2 [1.88 GB] / in no VG: 1 [964.84 MB]
You can define a filter in the lvm.conf
so that this command will avoid scanning specific physical volumes. For
information on using filters to control which devices are scanned
c)Scanning for Block Devices:- You can scan for block devices that may be used as physical volumes with the lvmdiskscan command
d) Preventing Allocation on a Physical Volume
You can prevent allocation of physical extents on the free space of one or more physical volumes with the pvchange command. This may be necessary if there are disk errors, or if you will be removing the physical volume.
The following command disallows the allocation of physical extents on /dev/sdk1.
pvchange -x n /dev/sdk1
You can also use the -xy arguments of the pvchange command to allow allocation where it had previously been disallowed.
e) Removing Physical Volumes:-
If a device is no longer required for use by LVM, you can remove the LVM label with the pvremove command. Executing the pvremove command zeroes the LVM metadata on an empty physical volume.
If
the physical volume you want to remove is currently part of a volume
group, you must remove it from the volume group with the vgreduce command
pvremove /dev/ram15
Labels on physical volume "/dev/ram15" successfully wiped
Volume Group Administration:-
a) Creating Volume Groups:- To create a volume group from one or more physical volumes, use the vgcreate command. The vgcreate command creates a new volume group by name and adds at least one physical volume to it.
The following command creates a volume group named vg1 that contains physical volumes /dev/sdd1 and /dev/sde1.
vgcreate vg1 /dev/sdd1 /dev/sde1
When
physical volumes are used to create a volume group, its disk space is
divided into 4MB extents, by default. This extent is the minimum amount
by which the logical volume may be increased or decreased in size. Large
numbers of extents will have no impact on I/O performance of the
logical volume.
vgcreate -s 16m vg1 /dev/sdd1 /dev/sde1
You can put limits on the number of physical or logical volumes the volume group can have by using the -p and -l arguments of the vgcreate command.
By default, a volume group allocates physical extents according to
common-sense rules such as not placing parallel stripes on the same
physical volume. This is the normal allocation policy. You can use the --alloc argument of the vgcreate command to specify an allocation policy of contiguous, anywhere, or cling.
The contiguous
policy requires that new extents are adjacent to existing extents. If
there are sufficient free extents to satisfy an allocation request but a
normal allocation policy would not use them, the anywhere allocation policy will, even if that reduces performance by placing two stripes on the same physical volume. The cling
policy places new extents on the same physical volume as existing
extents in the same stripe of the logical volume. These policies can be
changed using the vgchange command.
In general, allocation policies other than normal are required only in special cases where you need to specify unusual or nonstandard extent allocation.
LVM volume groups and underlying logical volumes are included in the device special file directory tree in the /dev directory with the following layout:
/dev/vg/lv/
b) Adding Physical Volumes to a Volume Group:- To add additional physical volumes to an existing volume group, use the vgextend command. The vgextend command increases a volume group's capacity by adding one or more free physical volumes.
The following command adds the physical volume /dev/sdf1 to the volume group vg1.
vgextend vg1 /dev/sdf
c) Displaying Volume Groups
There are two commands you can use to display properties of LVM volume groups: vgs and vgdisplay.
The vgscan
command, which scans all the disks for volume groups and rebuilds the
LVM cache file, also displays the volume groups. For information on the vgscan command.
The vgs command provides volume group information in a configurable form, displaying one line per volume group. The vgs command provides a great deal of format control, and is useful for scripting. For information on using the vgs command to customize your output.
The vgdisplay
command displays volume group properties (such as size, extents, number
of physical volumes, etc.) in a fixed form. The following example shows
the output of a vgdisplay command for the volume group new_vg. If you do not specify a volume group, all existing volume groups are displayed.
# vgdisplay new_vg
--- Volume group ---
VG Name new_vg
System ID
Format lvm2
Metadata Areas 3
Metadata Sequence No 11
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 1
Open LV 0
Max PV 0
Cur PV 3
Act PV 3
VG Size 51.42 GB
PE Size 4.00 MB
Total PE 13164
Alloc PE / Size 13 / 52.00 MB
Free PE / Size 13151 / 51.37 GB
VG UUID jxQJ0a-ZKk0-OpMO-0118-nlwO-wwqd-fD5D32
d) Scanning Disks for Volume Groups to Build the Cache File:-
The vgscan
command scans all supported disk devices in the system looking for LVM
physical volumes and volume groups. This builds the LVM cache in the /etc/lvm/.cache file, which maintains a listing of current LVM devices.
LVM runs the vgscan command automatically at system startup and at other times during LVM operation, such as when you execute a vgcreate command or when LVM detects an inconsistency. \
Note: -You may need to run the vgscan
command manually when you change your hardware configuration and add or
delete a device from a node, causing new devices to be visible to the
system that were not present at system bootup. This may be necessary,
for example, when you add new disks to the system on a SAN or hotplug a
new disk that has been labeled as a physical volume.
vgscan
Reading all physical volumes. This may take a while...
Found volume group "new_vg" using metadata type lvm2
Found volume group "officevg" using metadata type lvm2
e)Removing Physical Volumes from a Volume Group
To remove unused physical volumes from a volume group, use the vgreduce command. The vgreduce command shrinks a volume group's capacity by removing one or more empty physical volumes.
If PV has some PE allocated to LV then will have to migrate data to another PV.
Ex: pvmove /dev/sdb1 /dev/sdc1
vgreduce /dev/sdb1
Before removing a physical volume from a volume group, you can
make sure that the physical volume is not used by any logical volumes by
using the pvdisplay command.
# pvdisplay /dev/hda1
-- Physical volume ---
PV Name /dev/hda1
VG Name myvg
PV Size 1.95 GB / NOT usable 4 MB [LVM: 122 KB]
PV# 1
PV Status available
Allocatable yes (but full)
Cur LV 1
PE Size (KByte) 4096
Total PE 499
Free PE 0
Allocated PE 499
PV UUID Sd44tK-9IRw-SrMC-MOkn-76iP-iftz-OVSen7
If the physical volume is still being used you will have to migrate the data to another physical volume using the pvmove command. Then use the vgreduce command to remove the physical volume:
The following command removes the physical volume /dev/sda1 from the volume group my_volume_group.
vgreduce my_volume_group /dev/sda1
f)Changing the Parameters of a Volume Group:-
The
vgchange command is used to deactivate and activate volume groups, as
described in You can also use this command to change several volume
group parameters for an existing volume group.
The following command changes the maximum number of logical volumes of volume group vg00 to 128.
vgchange -l 128 /dev/vg00
The following example deactivates the volume group my_volume_group.
vgchange -a n my_volume_group
For a description of the volume group parameters you can change with the vgchange command, see the vgchange(8) man page.
g) Removing Volume Groups:-
To remove a volume group that contains no logical volumes, use the vgremove command.
# vgremove officevg
Volume group "officevg" successfully removed
h)Splitting a Volume Group :-
To split the physical volumes of a volume group and create a new volume group, use the vgsplit command.
Logical
volumes cannot be split between volume groups. Each existing logical
volume must be entirely on the physical volumes forming either the old
or the new volume group. If necessary, however, you can use the pvmove command to force the split.
The following example splits off the new volume group smallvg from the original volume group bigvg.
# vgsplit bigvg smallvg /dev/ram15
Volume group "smallvg" successfully split from "bigvg"
Note:- If LV is mounted, then unmout LV, deactivate both first LV and then VG
I) Combining Volume Groups:-
Two combine two volume groups into a single volume group, use the vgmerge
command. You can merge an inactive "source" volume with an active or an
inactive "destination" volume if the physical extent sizes of the
volume are equal and the physical and logical volume summaries of both
volume groups fit into the destination volume groups limits.
The following command merges the inactive volume group my_vg into the active or inactive volume group databases giving verbose runtime information.
vgmerge -v databases my_vg
j) Backing Up Volume Group Metadata :-
Metadata backups and archives are automatically created on every volume
group and logical volume configuration change unless disabled in the lvm.conf file. By default, the metadata backup is stored in the /etc/lvm/backup file and the metadata archives are stored in the /etc/lvm/archives file. You can manually back up the metadata to the /etc/lvm/backup file with the vgcfgbackup
command.
Ex: vgcfgbackup or
vgcfgbackup volume_gruoup_name
The vgcfrestore command restores the metadata of a volume group from the archive to all the physical volumes in the volume groups.
k) Renaming a Volume Group :-
Use the vgrename command to rename an existing volume group.
Either of the following commands renames the existing volume group vg02 to my_volume_group
vgrename /dev/vg02 /dev/my_volume_group
vgrename vg02 my_volume_group
l) Moving a Volume Group to Another System
You can move an entire LVM volume group to another system. It is recommended that you use the vgexport and vgimport commands when you do this.
The vgexport command makes an inactive volume group inaccessible to the system, which allows you to detach its physical volumes. The vgimport command makes a volume group accessible to a machine again after the vgexport command has made it inactive.
To move a volume group form one system to another, perform the following steps:
Make sure that no users are accessing files on the active volumes in the volume group, then unmount the logical volumes.
Use the -a n argument of the vgchange -a n command to mark the volume group as inactive, which prevents any further activity on the volume group.
Use the vgexport vg_name command to export the volume group. This prevents it from being accessed by the system from which you are removing it.
After you export the volume group, the physical volume will show up as being in an exported volume group when you execute the pvscan command, as in the following example.
[root@tng3-1]# pvscan
PV /dev/sda1 is in exported VG myvg [17.15 GB / 7.15 GB free]
PV /dev/sdc1 is in exported VG myvg [17.15 GB / 15.15 GB free]
PV /dev/sdd1 is in exported VG myvg [17.15 GB / 15.15 GB free]
...
When
the system is next shut down, you can unplug the disks that constitute
the volume group and connect them to the new system.
When the disks are plugged into the new system, use the vgimport command to import the volume group, making it accessible to the new system.
Activate the volume group with the -a y argument of the vgchange command.
- Mount the file system to make it available for use.
m) Recreating a Volume Group Directory :-
To recreate a volume group directory and logical volume special files, use the vgmknodes command. This command checks the LVM2 special files in the /dev directory that are needed for active logical volumes. It creates any special files that are missing removes unused ones.
You can incorporate the vgmknodes command into the vgscan command by specifying the mknodes argument to the vgscan command.
Logical Volume Administration.
Creating Logical Volumes :-
To create a logical volume, use the lvcreate command. You can create linear volumes, striped volumes, and mirrored volumes, as described in the following subsections.
If you do not specify a name for the logical volume, the default name lvol# is used where # is the internal number of the logical volume.
The
following sections provide examples of logical volume creation for the
three types of logical volumes you can create with LVM.
Note
: - The allocation policy to use: contiguous, cling, normal, anywhere
or inherit. When a command needs to allocate physical extents from
the volume group, the allocation policy controls how they are chosen.
Each volume group and logical volume has an allocation policy. The
default for a
volume group is normal which applies common-sense rules such as not placing parallel stripes on
the same physical volume. The default for a logical volume is inherit which applies the same pol-
icy as for the volume group. These policies can be changed using lvchange (8) and vgchange (8) or
over-ridden on the command line of any command that performs allocation. The contiguous policy
requires that new extents be placed adjacent to existing extents. The cling policy places new
extents on the same physical volume as existing extents in the same stripe of the Logical Volume.
If there are sufficient free extents to satisfy an allocation request but normal doesn’t use them,
anywhere will - even if that reduces performance by placing two stripes on the same physical volume.
a) Creating Linear Volumes :-
When you create a logical volume, the logical volume is carved from a
volume group using the free extents on the physical volumes that make up
the volume group. Normally logical volumes use up any space available
on the underlying physical volumes on a next-free basis. Modifying the
logical volume frees and reallocates space in the physical volumes.
The following command creates a logical volume 10 gigabytes in size in the volume group vg1.
lvcreate -L 10G vg1
The following command creates a 1500 megabyte linear logical volume named testlv in the volume group testvg, creating the block device /dev/testvg/testlv.
lvcreate -L1500 -n testlv testvg
The following command creates a 50 gigabyte logical volume named gfslv from the free extents in volume group vg0.
lvcreate -L 50G -n gfslv vg0
You can use the -l argument of the lvcreate
command to specify the size of the logical volume in extents. You can
also use this argument to specify the percentage of the volume group to
use for the logical volume. The following command creates a logical
volume called mylv that uses 60% of the total space in volume group testvol.
lvcreate -l 60%VG -n mylv testvg
You can also use the -l argument of the lvcreate
command to specify the percentage of the remaining free space in a
volume group as the size of the logical volume. The following command
creates a logical volume called yourlv that uses all of the unallocated space in the volume group testvol.
lvcreate -l 100%FREE -n yourlv testvg
You can use -l argument of the lvcreate
command to create a logical volume that uses the entire volume group.
Another way to create a logical volume that uses the entire volume group
is to use the vgdisplay command to find the "Total PE" size and to use those results as input to the the lvcreate command.
The following commands create a logical volume called mylv that fills the volume group named testvg.
# vgdisplay testvg | grep "Total PE"
Total PE 10230
# lvcreate -l 10230 testvg -n mylv
The underlying physical volumes used to create a logical volume can be
important if the physical volume needs to be removed, so you may need to
consider this possibility when you create the logical volume. For
information on removing a physical volume from a volume group.
To
create a logical volume to be allocated from a specific physical volume
in the volume group, specify the physical volume or volumes at the end
at the lvcreate command line. The following command creates a logical volume named testlv in volume group testvg allocated from the physical volume /dev/sdg1,
lvcreate -L 1500 -ntestlv testvg /dev/sdg1
You
can specify which extents of a physical volume are to be used for a
logical volume. The following example creates a linear logical volume
out of extents 0 through 25 of physical volume /dev/sda1 and extents 50 through 125 of physical volume /dev/sdb1 in volume group testvg.
lvcreate -l 100 -n testlv testvg /dev/sda1:0-25 /dev/sdb1:50-125
The following example creates a linear logical volume out of extents 0 through 25 of physical volume /dev/sda1 and then continues laying out the logical volume at extent 100.
lvcreate -l 100 -n testlv testvg /dev/sda1:0-25:100-
The default policy for how the extents of a logical volume are allocated is inherit, which applies the same policy as for the volume group. These policies can be changed using the lvchange command. For information on allocation policies
b) Creating Striped Volumes:-
For large sequential reads and writes, creating a striped logical
volume can improve the efficiency of the data I/O. For general
information about striped volumes. When you create a striped logical
volume, you specify the number of stripes with the -i argument of the lvcreate
command. This determines over how many physical volumes the logical
volume will be striped. The number of stripes cannot be greater than the
number of physical volumes in the volume group (unless the --alloc anywhere argument is used).
If
the underlying physical devices that make up a striped logical volume
are different sizes, the maximum size of the striped volume is
determined by the smallest underlying device. For example, in a
two-legged stripe, the maximum size is twice the size of the smaller
device. In a three-legged stripe, the maximum size is three times the
size of the smallest device.
The following command creates a
striped logical volume across 2 physical volumes with a stripe of 64kB.
The logical volume is 50 gigabytes in size, is named gfslv, and is carved out of volume group vg0.
lvcreate -L 50G -i2 -I64 -n gfslv vg0
As
with linear volumes, you can specify the extents of the physical volume
that you are using for the stripe. The following command creates a
striped volume 100 extents in size that stripes across two physical
volumes, is named stripelv and is in volume group testvg. The stripe will use sectors 0-50 of /dev/sda1 and sectors 50-100 of /dev/sdb1.
# lvcreate -l 100 -i2 -nstripelv testvg /dev/sda1:0-50 /dev/sdb1:50-100
Using default stripesize 64.00 KB
Logical volume "stripelv" created
To view how stripe is created use this command
lvs -a -o +stripes,stripesize,devices
LV VG Attr LSize Origin Snap% Move Log Copy% #Str Stripe Devices
pav vada -wi-ao 484.00M 1 0 /dev/sda5(0)
vol1 vg -wi-a- 640.00M 2 64.00K /dev/sdb1(0),/dev/sdc1(0)
vol2 vg -wi-a- 192.00M 2 64.00K /dev/sdb1(20),/dev/sdc1(20)
c) Creating Mirrored Volumes :-
When you create a mirrored volume, you specify the number of copies of the data to make with the -m argument of the lvcreate command. Specifying -m1 creates one mirror, which yields two copies of the file system: a linear logical volume plus one copy. Similarly, specifying -m2 creates two mirrors, yielding three copies of the file system.
The following command creates a mirrored logical volume with a single mirror. The volume is 50 gigabytes in size, is named mirrorlv, and is carved out of volume group vg0:
lvcreate -L 50G -m1 -n mirrorlv vg0
An LVM mirror divides the device being copied into regions that, by default, are 512KB in size. You can use the -R argument of the lvcreate command to specify the region size in MB. You can also change the default region size by editing the mirror_region_size setting in the lvm.conf file.
Note :-
Due
to limitations in the cluster infrastructure, cluster mirrors greater
than 1.5TB cannot be created with the default region size of 512KB.
Users that require larger mirrors should increase the region size from
its default to something larger. Failure to increase the region size
will cause LVM creation to hang and may hang other LVM commands as well.
As
a general guideline for specifying the region size for mirrors that are
larger than 1.5TB, you could take your mirror size in terabytes and
round up that number to the next power of 2, using that number as the -R argument to the lvcreate command. For example, if your mirror size is 1.5TB, you could specify -R 2. If your mirror size is 3TB, you could specify -R 4. For a mirror size of 5TB, you could specify -R 8.
The following command creates a mirrored logical volume with a region size of 2MB:
lvcreate -m1 -L 2T -R 2 -n mirror vol_group
LVM
maintains a small log which it uses to keep track of which regions are
in sync with the mirror or mirrors. By default, this log is kept on
disk, which keeps it persistent across reboots. You can specify instead
that this log be kept in memory with the --corelog
argument; this eliminates the need for an extra log device, but it
requires that the entire mirror be resynchronized at every reboot.
The following command creates a mirrored logical volume from the volume group bigvg. The logical is named ondiskmirvol and has a single mirror. The volume is 12MB in size and keeps the mirror log in memory.
# lvcreate -L 12MB -m1 --corelog -n ondiskmirvol bigvg
Logical volume "ondiskmirvol" created
The mirror log is created on a separate device from the devices on
which any of the mirror legs are created. It is possible, however, to
create the mirror log on the same device as one of the mirror legs by
using the --alloc anywhere argument of the vgcreate command. This may degrade performance, but it allows you to create a mirror even if you have only two underlying devices.
The
following command creates a mirrored logical volume with a single
mirror for which the mirror log is on the same device as one of the
mirror legs. In this example, the volume group vg0 consists of only two devices. This command creates a 500 megabyte volume named mirrorlv in the vg0 volume group.
lvcreate -L 500M -m1 -n mirrorlv -alloc anywhere vg0
When a mirror is created, the mirror regions are synchronized. For
large mirror components, the sync process may take a long time. When you
are creating a new mirror that does not need to be revived, you can
specify the nosync argument to indicate that an initial synchronization from the first device is not required.
You
can specify which devices to use for the mirror logs and log, and which
extents of the devices to use. To force the log onto a particular disk,
specify exactly one extent on the disk on which it will be placed. LVM
does not necessary respect the order in which devices are listed in the
command line. If any physical volumes are listed that is the only space
on which allocation will take place. Any physical extents included in
the list that are already allocated will get ignored.
The following command creates a mirrored logical volume with a single mirror. The volume is 500 megabytes in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on device /dev/sda1, the second leg of the mirror is on device /dev/sdb1, and the mirror log is on /dev/sdc1.
lvcreate -L 500M -m1 -n mirrorlv vg0 /dev/sda1 /dev/sdb1 /dev/sdc1
The following command creates a mirrored logical volume with a single mirror. The volume is 500 megabytes in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on extents 0 through 499 of device /dev/sda1, the second leg of the mirror is on extents 0 through 499 of device /dev/sdb1, and the mirror log starts on extent 0 of device /dev/sdc1. These are 1MB extents. If any of the specified extents have already been allocated, they will be ignored.
lvcreate -L 500M -m1 -n mirrorlv vg0 /dev/sda1:0-499 /dev/sdb1:0-499 /dev/sdc1:0
d)Changing Mirrored Volume Configuration:-
You can convert a logical volume from a mirrored volume to a linear
volume or from a linear volume to a mirrored volume with the lvconvert command. You can also use this command to reconfigure other mirror parameters of an existing logical volume, such as corelog.
When
you convert a logical volume to a mirrored volume, you are basically
creating mirror legs for an existing volume. This means that your volume
group must contain the devices and space for the mirror legs and for
the mirror log.
If you lose a
leg of a mirror, LVM converts the volume to a linear volume so that you
still have access to the volume, without the mirror redundancy. After
you replace the leg, you can use the lvconvert command to restore the mirror.
The following command converts the linear logical volume vg00/lvol1 to a mirrored logical volume.
lvconvert -m1 vg00/lvol1
The following command converts the mirrored logical volume vg00/lvol1 to a linear logical volume, removing the mirror leg.
lvconvert -m0 vg00/lvol1
e)Persistent Device Numbers:-
Major and minor device numbers are allocated dynamically at module
load. Some applications work best if the block device always is
activated with the same device (major and minor) number. You can specify
these with the lvcreate and the lvchange commands by using the following arguments:
--persistent y --major major --minor minor
Use a large minor number to be sure that it hasn't already been allocated to another device dynamically.
If you are exporting a file system using NFS, specifying the fsid parameter in the exports file may avoid the need to set a persistent device number within LVM.
f) Changing the Parameters of a Logical Volume Group:-
To change the parameters of a logical volume, use the lvchange command. For a listing of the parameters you can change, see the lvchange(8) man page.
You can use the lvchange
command to activate and deactivate logical volumes. To activate and
deactivate all the logical volumes in a volume group at the same time,
use the vgchange command.
The following command changes the permission on volume lvol1 in volume group vg00 to be read-only.
lvchange -pr vg00/lvol1
g) Renaming Logical Volumes:-
To rename an existing logical volume, use the lvrename command.
Either of the following commands renames logical volume lvold in volume group vg02 to lvnew.
lvrename /dev/vg02/lvold /dev/vg02/lvnew
lvrename vg02 lvold lvnew
h) Removing Logical Volumes:-
To remove an inactive logical volume, use the lvremove
command. If the logical volume is currently mounted, unmount the volume
before removing it. In addition, in a clustered environment you must
deactivate a logical volume before it can be removed.
The following command removes the logical volume /dev/testvg/testlv. from the volume group testvg. Note that in this case the logical volume has not been deactivated.
[root@tng3-1 lvm]# lvremove /dev/testvg/testlv
Do you really want to remove active logical volume "testlv"? [y/n]: y
Logical volume "testlv" successfully removed
You could explicitly deactivate the logical volume before removing it with the lvchange -an command, in which case you would not see the prompt verifying whether you want to remove an active logical volume.
i) Growing Logical Volumes:-
To increase the size of a logical volume, use the lvextend command.
After extending the logical volume, you will need to increase the size of the associated file system to match.
When
you extend the logical volume, you can indicate how much you want to
extend the volume, or how large you want it to be after you extend it.
The following command extends the logical volume /dev/myvg/homevol to 12 gigabytes.
# lvextend -L12G /dev/myvg/homevol
lvextend -- extending logical volume "/dev/myvg/homevol" to 12 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended
The following command adds another gigabyte to the logical volume /dev/myvg/homevol.
# lvextend -L+1G /dev/myvg/homevol
lvextend -- extending logical volume "/dev/myvg/homevol" to 13 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended
As with the lvcreate command, you can use the -l argument of the lvextend
command to specify the number of extents by which to increase the size
of the logical volume. You can also use this argument to specify a
percentage of the volume group, or a percentage of the remaining free
space in the volume group. The following command extends the logical
volume called testlv to fill all of the unallocated space in the volume group myvg.
[root@tng3-1 ~]# lvextend -l +100%FREE /dev/myvg/testlv
Extending logical volume testlv to 68.59 GB
Logical volume testlv successfully resized
After you have extended the logical volume it is necessary to increase the file system size to match.
By
default, most file system resizing tools will increase the size of the
file system to be the size of the underlying logical volume so you do
not need to worry about specifying the same size for each of the two
commands.
j) Extending a Striped Volume:-
In order to increase the size of a striped logical volume, there must
be enough free space on the underlying physical volumes that make up the
volume group to support the stripe. For example, if you have a two-way
stripe that that uses up an entire volume group, adding a single
physical volume to the volume group will not enable you to extend the
stripe. Instead, you must add at least two physical volumes to the
volume group.
For example, consider a volume group vg that consists of two underlying physical volumes, as displayed with the following vgs command.
# vgs
VG #PV #LV #SN Attr VSize VFree
vg 2 0 0 wz--n- 271.31G 271.31G
You can create a stripe using the entire amount of space in the volume group.
# lvcreate -n stripe1 -L 271.31G -i 2 vg
Using default stripesize 64.00 KB
Rounding up size to full physical extent 271.31 GB
Logical volume "stripe1" created
# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
stripe1 vg -wi-a- 271.31G /dev/sda1(0),/dev/sdb1(0)
Note that the volume group now has no more free space.
# vgs
VG #PV #LV #SN Attr VSize VFree
vg 2 1 0 wz--n- 271.31G 0
The following command adds another physical volume to the volume group, which then has 135G of additional space.
# vgextend vg /dev/sdc1
Volume group "vg" successfully extended
# vgs
VG #PV #LV #SN Attr VSize VFree
vg 3 1 0 wz--n- 406.97G 135.66G
At this point you cannot
extend the striped logical volume to the full size of the volume group,
because two underlying devices are needed in order to stripe the data.
# lvextend vg/stripe1 -L 406G
Using stripesize of last segment 64.00 KB
Extending logical volume stripe1 to 406.00 GB
Insufficient suitable allocatable extents for logical volume stripe1: 34480
more required
To extend the striped logical volume, add another
physical volume and then extend the logical volume. In this example,
having added two physical volumes to the volume group we can extend the
logical volume to the full size of the volume group.
# vgextend vg /dev/sdd1
Volume group "vg" successfully extended
# vgs
VG #PV #LV #SN Attr VSize VFree
vg 4 1 0 wz--n- 542.62G 271.31G
# lvextend vg/stripe1 -L 542G
Using stripesize of last segment 64.00 KB
Extending logical volume stripe1 to 542.00 GB
Logical volume stripe1 successfully resized
If you do not have
enough underlying physical devices to extend the striped logical volume,
it is possible to extend the volume anyway if it does not matter that
the extension is not striped, which may result in uneven performance.
When adding space to the logical volume, the default operation is to use
the same striping parameters of the last segment of the existing
logical volume, but you can override those parameters. The following
example extends the existing striped logical volume to use the remaining
free space after the initial lvextend command fails.
# lvextend vg/stripe1 -L 406G
Using stripesize of last segment 64.00 KB
Extending logical volume stripe1 to 406.00 GB
Insufficient suitable allocatable extents for logical volume stripe1: 34480
more required
# lvextend -i1 -l+100%FREE vg/stripe1
k) Shrinking Logical Volumes:-
To reduce the size of a logical volume, first unmount the file system. You can then use the lvreduce command to shrink the volume. After shrinking the volume, remount the file system.
The following example reduces the size of logical volume lvol1 in volume group vg00 by 3 logical extents.
lvreduce -l -3 vg00/lvol1
Creating Snapshot Volumes :-
Use the -s argument of the lvcreate command to create a snapshot volume. A snapshot volume is writeable.
Note :-
LVM snapshots are not supported across the nodes in a cluster. You cannot create a snapshot volume in a clustered volume group.
The following command creates a snapshot logical volume that is 100 megabytes in size named /dev/vg00/snap. This creates a snapshot of the origin logical volume named /dev/vg00/lvol1.
If the original logical volume contains a file system, you can mount
the snapshot logical volume on an arbitrary directory in order to access
the contents of the file system to run a backup while the original file
system continues to get updated.
lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
After you create a snapshot logical volume, specifying the origin volume on the lvdisplay command yields output that includes a a list of all snapshot logical volumes and their status (active or inactive).
The following example shows the status of the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created.
# lvdisplay /dev/new_vg/lvol0
--- Logical volume ---
LV Name /dev/new_vg/lvol0
VG Name new_vg
LV UUID LBy1Tz-sr23-OjsI-LT03-nHLC-y8XW-EhCl78
LV Write Access read/write
LV snapshot status source of
/dev/new_vg/newvgsnap1 [active]
LV Status available
# open 0
LV Size 52.00 MB
Current LE 13
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:2
The lvs
command, by default, displays the origin volume and the current
percentage of the snapshot volume being used for each snapshot volume.
The following example shows the default output for the lvs command for a system that includes the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created.
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
lvol0 new_vg owi-a- 52.00M
newvgsnap1 new_vg swi-a- 8.00M lvol0 0.20
Note
Because
the snapshot increases in size as the origin volume changes, it is
important to monitor the percentage of the snapshot volume regularly
with the lvs command to be sure it does not
fill. A snapshot that is 100% full is lost completely, as a write to
unchanged parts of the origin would be unable to succeed without
corrupting the snapshot.
Online Data Relocation : -
You can move data while the system is in use with the pvmove command.
The pvmove
command breaks up the data to be moved into sections and creates a
temporary mirror to move each section. For more information on the
operation of the pvmove command, see the pvmove(8) man page.
Because the pvmove command uses mirroring, it is not cluster-aware and needs exclusive access to a volume.
The following command moves all allocated space off the physical volume /dev/sdc1 to other free physical volumes in the volume group:
pvmove /dev/sdc1
The following command moves just the extents of the logical volume MyLV.
pvmove -n MyLV /dev/sdc1
Since the pvmove
command can take a long time to execute, you may want to run the
command in the background to avoid display of progress updates in the
foreground. The following command moves all extents allocated to to the
physical volume /dev/sdc1 over to /dev/sdf1 in the background.
pvmove -b /dev/sdc1 /dev/sdf1
The following command reports the progress of the move as a percentage at five second intervals.
pvmove -i5 /dev/sdd1
Customized Reporting for LVM
You can produce concise and customizable reports of LVM objects with the pvs, lvs, and vgs
commands. The reports that these commands generate include one line of
output for each object. Each line contains an ordered list of fields of
properties related to the object. There are five ways to select the
objects to be reported: by physical volume, volume group, logical
volume, physical volume segment, and logical volume segment.
The following sections provide:
A summary of command arguments you can use to control the format of the generated report.
A list of the fields you can select for each LVM object.
A summary of command arguments you can use to sort the generated report.
- Instructions for specifying the units of the report output.
a) Format Control : -
Whether you use the pvs, lvs, or vgs
command determines the default set of fields displayed and the sort
order. You can control the output of these commands with the following
arguments:
- You can change what fields are displayed to something other than the default by using the
-o argument. For example, the following output is the default display for the pvs command (which displays information about physcial volumes).
# pvs
PV VG Fmt Attr PSize PFree
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G
/dev/sdd1 new_vg lvm2 a- 17.14G 17.14G
The following command displays only the physical volume name and size.
# pvs -o pv_name,pv_size
PV PSize
/dev/sdb1 17.14G
/dev/sdc1 17.14G
/dev/sdd1 17.14G
The following example displays the UUID of the physical volume in addition to the default fields.
# pvs -o +pv_uuid
PV VG Fmt Attr PSize PFree PV UUID
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G Joqlch-yWSj-kuEn-IdwM-01S9-X08M-mcpsVe
/dev/sdd1 new_vg lvm2 a- 17.14G 17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-UqkCS
- Adding the
-v argument to a command includes some extra fields. For example, the pvs -v command will display the DevSize and PV UUID fields in addition to the default fields.
# pvs -v
Scanning for physical volume names
PV VG Fmt Attr PSize PFree DevSize PV UUID
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G 17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G 17.14G Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe
/dev/sdd1 new_vg lvm2 a- 17.14G 17.14G 17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-tUqkCS
The following example uses the --noheadings argument in combination with the pv_name argument, which will generate a list of all physical volumes.
# pvs --noheadings -o pv_name
/dev/sdb1
/dev/sdc1
/dev/sdd1
The following example separates the default output fields of the pvs command with an equals sign (=).
# pvs --separator =
PV=VG=Fmt=Attr=PSize=PFree
/dev/sdb1=new_vg=lvm2=a-=17.14G=17.14G
/dev/sdc1=new_vg=lvm2=a-=17.14G=17.09G
/dev/sdd1=new_vg=lvm2=a-=17.14G=17.14G
To keep the fields aligned when using the separator argument, use the separator argument in conjunction with the --aligned argument.
# pvs --separator = --aligned
PV =VG =Fmt =Attr=PSize =PFree
/dev/sdb1 =new_vg=lvm2=a- =17.14G=17.14G
/dev/sdc1 =new_vg=lvm2=a- =17.14G=17.09G
/dev/sdd1 =new_vg=lvm2=a- =17.14G=17.14G
You can use the -P argument of the lvs or vgs command to display information about a failed volume that would otherwise not appear in the output.
For a full listing of display arguments, see the pvs(8), vgs(8) and lvs(8) man pages.
Volume
group fields can be mixed with either physical volume (and physical
volume segment) fields or with logical volume (and logical volume
segment) fields, but physical volume and logical volume fields cannot be
mixed. For example, the following command will display one line of
output for each physical volume.
# vgs -o +pv_name
VG #PV #LV #SN Attr VSize VFree PV
new_vg 3 1 0 wz--n- 51.42G 51.37G /dev/sdc1
new_vg 3 1 0 wz--n- 51.42G 51.37G /dev/sdd1
new_vg 3 1 0 wz--n- 51.42G 51.37G /dev/sdb1
b) Object Selection
This section provides a series of tables that list the information you can display about the LVM objects with the pvs, vgs, and lvs commands.
For convenience, a field name prefix can be dropped if it matches the default for the command. For example, with the pvs command, name means pv_name, but with the vgs command, name is interpreted as vg_name.
Executing the following command is the equivalent of executing pvs -o pv_free.
# pvs -o +free
PFree
17.14G
17.09G
17.14G
The pvs Command
the pvs command, along with the field name as it appears in the header display and a description of the field.
| Argument | Header | Description |
dev_size | DevSize | The size of the underlying device on which the physical volume was created |
pe_start | 1st PE | Offset to the start of the first physical extent in the underlying device |
pv_attr | Attr | Status of the physical volume: (a)llocatable or e(x)ported. |
pv_fmt | Fmt | The metadata format of the physical volume (lvm2 or lvm1) |
pv_free | PFree | The free space remaining on the physical volume |
pv_name | PV | The physical volume name |
pv_pe_alloc_count | Alloc | Number of used physical extents |
pv_pe_count | PE | Number of physical extents |
pvseg_size | SSize | The segment size of the physical volume |
pvseg_start | Start | The starting physical extent of the physical volume segment |
pv_size | PSize | The size of the physical volume |
pv_tags | PV Tags | LVM tags attached to the physical volume |
pv_used | Used | The amount of space currently used on the physical volume |
pv_uuid | PV UUID | The UUID of the physical volume |
Table 4.1. pvs Display Fields
The pvs command displays the following fields by default: pv_name, vg_name, pv_fmt, pv_attr, pv_size, pv_free. The display is sorted by pv_name.
# pvs
PV VG Fmt Attr PSize PFree
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G
/dev/sdd1 new_vg lvm2 a- 17.14G 17.13G
Using the -v argument with the pvs command adds the following fields to the default display: dev_size, pv_uuid.
# pvs -v
Scanning for physical volume names
PV VG Fmt Attr PSize PFree DevSize PV UUID
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G 17.14G onFF2w-1fLC-ughJ-D9eB-M7iv-6XqA-dqGeXY
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G 17.14G Joqlch-yWSj-kuEn-IdwM-01S9-XO8M-mcpsVe
/dev/sdd1 new_vg lvm2 a- 17.14G 17.13G 17.14G yvfvZK-Cf31-j75k-dECm-0RZ3-0dGW-tUqkCS
You can use the --segments argument of the pvs
command to display information about each physical volume segment. A
segment is a group of extents. A segment view can be useful if you want
to see whether your logical volume is fragmented.
The pvs --segments command displays the following fields by default: pv_name, vg_name, pv_fmt, pv_attr, pv_size, pv_free, pvseg_start, pvseg_size. The display is sorted by pv_name and pvseg_size within the physical volume.
# pvs --segments
PV VG Fmt Attr PSize PFree Start SSize
/dev/hda2 VolGroup00 lvm2 a- 37.16G 32.00M 0 1172
/dev/hda2 VolGroup00 lvm2 a- 37.16G 32.00M 1172 16
/dev/hda2 VolGroup00 lvm2 a- 37.16G 32.00M 1188 1
/dev/sda1 vg lvm2 a- 17.14G 16.75G 0 26
/dev/sda1 vg lvm2 a- 17.14G 16.75G 26 24
/dev/sda1 vg lvm2 a- 17.14G 16.75G 50 26
/dev/sda1 vg lvm2 a- 17.14G 16.75G 76 24
/dev/sda1 vg lvm2 a- 17.14G 16.75G 100 26
/dev/sda1 vg lvm2 a- 17.14G 16.75G 126 24
/dev/sda1 vg lvm2 a- 17.14G 16.75G 150 22
/dev/sda1 vg lvm2 a- 17.14G 16.75G 172 4217
/dev/sdb1 vg lvm2 a- 17.14G 17.14G 0 4389
/dev/sdc1 vg lvm2 a- 17.14G 17.14G 0 4389
/dev/sdd1 vg lvm2 a- 17.14G 17.14G 0 4389
/dev/sde1 vg lvm2 a- 17.14G 17.14G 0 4389
/dev/sdf1 vg lvm2 a- 17.14G 17.14G 0 4389
/dev/sdg1 vg lvm2 a- 17.14G 17.14G 0 4389
You can use the pvs -a command to see devices detected by LVM that have not been initialized as LVM physical volumes.
# pvs -a
PV VG Fmt Attr PSize PFree
/dev/VolGroup00/LogVol01 -- 0 0
/dev/new_vg/lvol0 -- 0 0
/dev/ram -- 0 0
/dev/ram0 -- 0 0
/dev/ram2 -- 0 0
/dev/ram3 -- 0 0
/dev/ram4 -- 0 0
/dev/ram5 -- 0 0
/dev/ram6 -- 0 0
/dev/root -- 0 0
/dev/sda -- 0 0
/dev/sdb -- 0 0
/dev/sdb1 new_vg lvm2 a- 17.14G 17.14G
/dev/sdc -- 0 0
/dev/sdc1 new_vg lvm2 a- 17.14G 17.09G
/dev/sdd -- 0 0
/dev/sdd1 new_vg lvm2 a- 17.14G 17.14G
The vgs Command
the vgs command, along with the field name as it appears in the header display and a description of the field.
| Argument | Header | Description |
lv_count | #LV | The number of logical volumes the volume group contains |
max_lv | MaxLV | The maximum number of logical volumes allowed in the volume group (0 if unlimited) |
max_pv | MaxPV | The maximum number of physical volumes allowed in the volume group (0 if unlimited) |
pv_count | #PV | The number of physical volumes that define the volume group |
snap_count | #SN | The number of snapshots the volume group contains |
vg_attr | Attr | Status of the volume group: (w)riteable, (r)eadonly, resi(z)eable, e(x)ported, (p)artial and (c)lustered. |
vg_extent_count | #Ext | The number of physical extents in the volume group |
vg_extent_size | Ext | The size of the physical extents in the volume group |
vg_fmt | Fmt | The metadata format of the volume group (lvm2 or lvm1) |
vg_free | VFree | Size of the free space remaining in the volume group |
vg_free_count | Free | Number of free physical extents in the volume group |
vg_name | VG | The volume group name |
vg_seqno | Seq | Number representing the revision of the volume group |
vg_size | VSize | The size of the volume group |
vg_sysid | SYS ID | LVM1 System ID |
vg_tags | VG Tags | LVM tags attached to the volume group |
vg_uuid | VG UUID | The UUID of the volume group |
Table 4.2. vgs Display Fields
The vgs command displays the following fields by default: vg_name, pv_count, lv_count, snap_count, vg_attr, vg_size, vg_free. The display is sorted by vg_name.
# vgs
VG #PV #LV #SN Attr VSize VFree
new_vg 3 1 1 wz--n- 51.42G 51.36G
Using the -v argument with the vgs command adds the following fields to the default display: vg_extent_size, vg_uuid.
# vgs -v
Finding all volume groups
Finding volume group "new_vg"
VG Attr Ext #PV #LV #SN VSize VFree VG UUID
new_vg wz--n- 4.00M 3 1 1 51.42G 51.36G jxQJ0a-ZKk0-OpMO-0118-nlwO-wwqd-fD5D32
The lvs Command
Table 4.3, “lvs Display Fields” lists the display arguments of the lvs command, along with the field name as it appears in the header display and a description of the field.
| Argument | Header | Description
|
| Chunk | Unit size in a snapshot volume |
copy_percent | Copy% | The synchronization percentage of a
mirrored logical volume; also used when physical extents are being
moved with the pv_move command |
devices | Devices | The underlying devices that make up the
logical volume: the physical volumes, logical volumes, and start
physical extents and logical extents |
lv_attr | Attr | The status of the logical volume. The logical volume attribute bits are as follows:
| Bit 1: Volume type: (m)irrored,
(M)irrored without initial sync, (o)rigin, (p)vmove, (s)napshot,
invalid (S)napshot, (v)irtual |
| Bit2: Permissions: (w)riteable, (r)ead-only |
Bit 3: Allocation policy:
(c)ontiguous, (n)ormal, (a)nywhere, (i)nherited. This is
capitalized if the volume is currently locked against allocation
changes, for example while executing the pvmove command. |
| Bit 4: fixed (m)inor |
| Bit 5 State: (a)ctive, (s)uspended,
(I)nvalid snapshot, invalid (S)uspended snapshot, mapped
(d)evice present without tables, mapped device present with
(i)nactive table |
| Bit 6: device (o)pen |
|
lv_kernel_major | KMaj | Actual major device number of the logical volume (-1 if inactive) |
lv_kernel_minor | KMIN | Actual minor device number of the logical volume (-1 if inactive) |
lv_major | Maj | The persistent major device number of the logical volume (-1 if not specified) |
lv_minor | Min | The persistent minor device number of the logical volume (-1 if not specified) |
lv_name | LV | The name of the logical volume |
lv_size | LSize | The size of the logical volume |
lv_tags | LV Tags | LVM tags attached to the logical volume |
lv_uuid | LV UUID | The UUID of the logical volume. |
mirror_log | Log | Device on which the mirror log resides |
modules | Modules | Corresponding kernel device-mapper target necessary to use this logical volume |
move_pv | Move | Source physical volume of a temporary logical volume created with the pvmove command |
origin | Origin | The origin device of a snapshot volume
|
| Region | The unit size of a mirrored logical volume |
seg_count | #Seg | The number of segments in the logical volume |
seg_size | SSize | The size of the segments in the logical volume |
seg_start | Start | Offset of the segment in the logical volume |
seg_tags | Seg Tags | LVM tags attached to the segments of the logical volume |
segtype | Type | The segment type of a logical volume (for example: mirror, striped, linear) |
snap_percent | Snap% | Current percentage of a snapshot volume that is in use |
stripes | #Str | Number of stripes or mirrors in a logical volume
|
| Stripe | Unit size of the stripe in a striped logical volume |
Table 4.3. lvs Display Fields
The lvs command displays the following fields by default: lv_name, vg_name, lv_attr, lv_size, origin, snap_percent, move_pv, mirror_log, copy_percent. The default display is sorted by vg_name and lv_name within the volume group.
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
lvol0 new_vg owi-a- 52.00M
newvgsnap1 new_vg swi-a- 8.00M lvol0 0.20
Using the -v argument with the lvs command adds the following fields to the default display: seg_count, lv_major, lv_minor, lv_kernel_major, lv_kernel_minor, lv_uuid.
# lvs -v
Finding all logical volumes
LV VG #Seg Attr LSize Maj Min KMaj KMin Origin Snap% Move Copy% Log LV UUID
lvol0 new_vg 1 owi-a- 52.00M -1 -1 253 3 LBy1Tz-sr23-OjsI-LT03-nHLC-y8XW-EhCl78
newvgsnap1 new_vg 1 swi-a- 8.00M -1 -1 253 5 lvol0 0.20 1ye1OU-1cIu-o79k-20h2-ZGF0-qCJm-CfbsIx
You can use the --segments argument of the lvs command to display information with default columns that emphasize the segment information. When you use the segments argument, the seg prefix is optional. The lvs --segments command displays the following fields by default: lv_name, vg_name, lv_attr, stripes, segtype, seg_size. The default display is sorted by vg_name, lv_name within the volume group, and seg_start within the logical volume. If the logical volumes were fragmented, the output from this command would show that.
# lvs --segments
LV VG Attr #Str Type SSize
LogVol00 VolGroup00 -wi-ao 1 linear 36.62G
LogVol01 VolGroup00 -wi-ao 1 linear 512.00M
lv vg -wi-a- 1 linear 104.00M
lv vg -wi-a- 1 linear 104.00M
lv vg -wi-a- 1 linear 104.00M
lv vg -wi-a- 1 linear 88.00M
Using the -v argument with the lvs --segments command adds the following fields to the default display: seg_start, stripesize, chunksize.
# lvs -v --segments
Finding all logical volumes
LV VG Attr Start SSize #Str Type Stripe Chunk
lvol0 new_vg owi-a- 0 52.00M 1 linear 0 0
newvgsnap1 new_vg swi-a- 0 8.00M 1 linear 0 8.00K
The following example shows the default output of the lvs command on a system with one logical volume configured, followed by the default output of the lvs command with the segments argument specified.
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
lvol0 new_vg -wi-a- 52.00M
# lvs --segments
LV VG Attr #Str Type SSize
lvol0 new_vg -wi-a- 1 linear 52.00M
c) Sorting LVM Reports :-
Normally the entire output of the lvs, vgs, or pvs
command has to be generated and stored internally before it can be
sorted and columns aligned correctly. You can specify the --unbuffered argument to display unsorted output as soon as it is generated.
To specify an alternative ordered list of columns to sort on, use the -O argument of any of the reporting commands. It is not necessary to include these fields within the output itself.
The following example shows the output of the pvs command that displays the physical volume name, size, and free space.
# pvs -o pv_name,pv_size,pv_free
PV PSize PFree
/dev/sdb1 17.14G 17.14G
/dev/sdc1 17.14G 17.09G
/dev/sdd1 17.14G 17.14G
The following example shows the same output, sorted by the free space field.
# pvs -o pv_name,pv_size,pv_free -O pv_free
PV PSize PFree
/dev/sdc1 17.14G 17.09G
/dev/sdd1 17.14G 17.14G
/dev/sdb1 17.14G 17.14G
The following example shows that you do not need to display the field on which you are sorting.
# pvs -o pv_name,pv_size -O pv_free
PV PSize
/dev/sdc1 17.14G
/dev/sdd1 17.14G
/dev/sdb1 17.14G
To display a reverse sort, precede a field you specify after the -O argument with the - character.
# pvs -o pv_name,pv_size,pv_free -O -pv_free
PV PSize PFree
/dev/sdd1 17.14G 17.14G
/dev/sdb1 17.14G 17.14G
/dev/sdc1 17.14G 17.09G
LVM Troubleshooting : -
a) troubleshooting Diagnostics
If a command is not working as expected, you can gather diagnostics in the following ways:
Use the -v, -vv, -vvv, or -vvvv argument of any command for increasingly verbose levels of output.
If the problem is related to the
logical volume activation, set 'activation = 1' in the 'log' section
of the configuration file and run the command with the -vvvv
argument. After you have finished examining this output be sure to
reset this parameter to 0, to avoid possible problems with the machine
locking during low memory situations.
Run the lvmdump command, which provides an information dump for diagnostic purposes. For information, see the lvmdump(8) man page.
Execute the lvs -v, pvs -a or “dmsetup info -c” command for additional system information.
Examine the last backup of the metadata in the /etc/lvm/backup file and archived versions in the /etc/lvm/archive file.
Check the current configuration information by running the lvm dumpconfig command.
- Check the
.cache file in the /etc/lvm directory for a record of which devices have physical volumes on them.
b) Displaying Information on Failed Devices :-
You can use the -P(provide access to volume groups that are only partially
available) argument of the lvs or vgs
command to display information about a failed volume that would
otherwise not appear in the output. This argument permits some
operations even though the metatdata is not completely consistent
internally. For example, if one of the devices that made up the volume
group vg failed, the vgs command might show the following output.
[root@link-07 tmp]# vgs -o +devices
Volume group "vg" not found
If you specify the -P argument of the vgs command, the volume group is still unusable but you can see more information about the failed device.
[root@link-07 tmp]# vgs -P -o +devices
Partial mode. Incomplete volume groups will be activated read-only.
VG #PV #LV #SN Attr VSize VFree Devices
vg 9 2 0 rz-pn- 2.11T 2.07T unknown device(0)
vg 9 2 0 rz-pn- 2.11T 2.07T unknown device(5120),/dev/sda1(0)
In this example, the failed device caused both a linear and a striped logical volume in the volume group to fail. The lvs command without the -P argument shows the following output.
[root@link-07 tmp]# lvs -a -o +devices
Volume group "vg" not found
Using the -P argument shows the logical volumes that have failed.
[root@link-07 tmp]# lvs -P -a -o +devices
Partial mode. Incomplete volume groups will be activated read-only.
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
linear vg -wi-a- 20.00G unknown device(0)
stripe vg -wi-a- 20.00G unknown device(5120),/dev/sda1(0)
The following examples show the output of the pvs and lvs commands with the -P argument specified when a leg of a mirrored logical volume has failed.
root@link-08 ~]# vgs -a -o +devices -P
Partial mode. Incomplete volume groups will be activated read-only.
VG #PV #LV #SN Attr VSize VFree Devices
corey 4 4 0 rz-pnc 1.58T 1.34T my_mirror_mimage_0(0),my_mirror_mimage_1(0)
corey 4 4 0 rz-pnc 1.58T 1.34T /dev/sdd1(0)
corey 4 4 0 rz-pnc 1.58T 1.34T unknown device(0)
corey 4 4 0 rz-pnc 1.58T 1.34T /dev/sdb1(0)
[root@link-08 ~]# lvs -a -o +devices -P
Partial mode. Incomplete volume groups will be activated read-only.
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
my_mirror corey mwi-a- 120.00G my_mirror_mlog 1.95 my_mirror_mimage_0(0),my_mirror_mimage_1(0)
[my_mirror_mimage_0] corey iwi-ao 120.00G unknown device(0)
[my_mirror_mimage_1] corey iwi-ao 120.00G /dev/sdb1(0)
[my_mirror_mlog] corey lwi-ao 4.00M /dev/sdd1(0)
c) Recovering from LVM Mirror Failure :-
This
section provides an example of recovering from a situation where one
leg of an LVM mirrored volume fails because the underlying device for a
physical volume goes down. When a mirror leg fails, LVM converts the
mirrored volume into a linear volume, which continues to operate as
before but without the mirrored redundancy. At that point, you can add a
new disk device to the system to use as a replacement physical device
and rebuild the mirror.
The following command creates the physical volumes which will be used for the mirror.
[root@link-08 ~]# pvcreate /dev/sd[abcdefgh][12]
Physical volume "/dev/sda1" successfully created
Physical volume "/dev/sda2" successfully created
Physical volume "/dev/sdb1" successfully created
Physical volume "/dev/sdb2" successfully created
Physical volume "/dev/sdc1" successfully created
Physical volume "/dev/sdc2" successfully created
Physical volume "/dev/sdd1" successfully created
Physical volume "/dev/sdd2" successfully created
Physical volume "/dev/sde1" successfully created
Physical volume "/dev/sde2" successfully created
Physical volume "/dev/sdf1" successfully created
Physical volume "/dev/sdf2" successfully created
Physical volume "/dev/sdg1" successfully created
Physical volume "/dev/sdg2" successfully created
Physical volume "/dev/sdh1" successfully created
Physical volume "/dev/sdh2" successfully created
The following commands creates the volume group vg and the mirrored volume groupfs.
[root@link-08 ~]# vgcreate vg /dev/sd[abcdefgh][12]
Volume group "vg" successfully creatlled
[root@link-08 ~]# lvcreate -L 750M -n groupfs -m 1 vg /dev/sda1 /dev/sdb1 /dev/sdc1
Rounding up size to full physical extent 752.00 MB
Logical volume "groupfs" created
You can use the lvs
command to verify the layout of the mirrored volume and the underlying
devices for the mirror leg and the mirror log. Note that in the first
example the mirror is not yet completely synced; you should wait until
the Copy% field displays 100.00 before continuing.
[root@link-08 ~]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
groupfs vg mwi-a- 752.00M groupfs_mlog 21.28 groupfs_mimage_0(0),groupfs_mimage_1(0)
[groupfs_mimage_0] vg iwi-ao 752.00M /dev/sda1(0)
[groupfs_mimage_1] vg iwi-ao 752.00M /dev/sdb1(0)
[groupfs_mlog] vg lwi-ao 4.00M /dev/sdc1(0)
[root@link-08 ~]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
groupfs vg mwi-a- 752.00M groupfs_mlog 100.00 groupfs_mimage_0(0),groupfs_mimage_1(0)
[groupfs_mimage_0] vg iwi-ao 752.00M /dev/sda1(0)
[groupfs_mimage_1] vg iwi-ao 752.00M /dev/sdb1(0)
[groupfs_mlog] vg lwi-ao 4.00M i /dev/sdc1(0)
In this example, the primary leg of the mirror /dev/sda1
fails. Any write activity to the mirrored volume causes LVM to detect
the failed mirror. When this occurs, LVM converts the mirror into a
single linear volume. In this case, to trigger the conversion, we
execute a dd command
[root@link-08 ~]# dd if=/dev/zero of=/dev/vg/groupfs count=10
10+0 records in
10+0 records out
You can use the lvs command to verify that the device is now a linear device. Because of the failed disk, I/O errors occur.
[root@link-08 ~]# lvs -a -o +devices
/dev/sda1: read failed after 0 of 2048 at 0: Input/output error
/dev/sda2: read failed after 0 of 2048 at 0: Input/output error
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
groupfs vg -wi-a- 752.00M /dev/sdb1(0)
At this point you should still be able to use the logical volume, but there will be no mirror redundancy.
To
rebuild the mirrored volume, you replace the broken drive and recreate
the physical volume. If you use the same disk rather than replacing it
with a new one, you will see "inconsistent" warnings when you run the pvcreate command.
[root@link-08 ~]# pvcreate /dev/sda[12]
Physical volume "/dev/sda1" successfully created
Physical volume "/dev/sda2" successfully created
[root@link-08 ~]# pvscan
PV /dev/sdb1 VG vg lvm2 [67.83 GB / 67.10 GB free]
PV /dev/sdb2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdc1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdc2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdd1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdd2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sde1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sde2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdf1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdf2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdg1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdg2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdh1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdh2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sda1 lvm2 [603.94 GB]
PV /dev/sda2 lvm2 [603.94 GB]
Total: 16 [2.11 TB] / in use: 14 [949.65 GB] / in no VG: 2 [1.18 TB]
Next you extend the original volume group with the new physical volume.
[root@link-08 ~]# vgextend vg /dev/sda[12]
Volume group "vg" successfully extended
[root@link-08 ~]# pvscan
PV /dev/sdb1 VG vg lvm2 [67.83 GB / 67.10 GB free]
PV /dev/sdb2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdc1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdc2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdd1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdd2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sde1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sde2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdf1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdf2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdg1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdg2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdh1 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sdh2 VG vg lvm2 [67.83 GB / 67.83 GB free]
PV /dev/sda1 VG vg lvm2 [603.93 GB / 603.93 GB free]
PV /dev/sda2 VG vg lvm2 [603.93 GB / 603.93 GB free]
Total: 16 [2.11 TB] / in use: 16 [2.11 TB] / in no VG: 0 [0 ]
Convert the linear volume back to its original mirrored state.
[root@link-08 ~]# lvconvert -m 1 /dev/vg/groupfs /dev/sda1 /dev/sdb1 /dev/sdc1
Logical volume mirror converted.
You can use the lvs command to verify that the mirror is restored.
[root@link-08 ~]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
groupfs vg mwi-a- 752.00M groupfs_mlog 68.62 groupfs_mimage_0(0),groupfs_mimage_1(0)
[groupfs_mimage_0] vg iwi-ao 752.00M /dev/sdb1(0)
[groupfs_mimage_1] vg iwi-ao 752.00M /dev/sda1(0)
[groupfs_mlog] vg lwi-ao 4.00M /dev/sdc1(0)
d) Recovering Physical Volume Metadata:-
If
the volume group metadata area of a physical volume is accidentally
overwritten or otherwise destroyed, you will get an error message
indicating that the metadata area is incorrect, or that the system was
unable to find a physical volume with a particular UUID. You may be
able to recover the data the physical volume by writing a new metadata
area on the physical volume specifying the same UUID as the lost
metadata.
Caution
You
should not attempt this procedure with a working LVM logical volume.
You will lose your data if you specify the incorrect UUID.
The following example shows the sort of output you may see if the metadata area is missing or corrupted.
[root@link-07 backup]# lvs -a -o +devices
Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
Couldn't find all physical volumes for volume group VG.
Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
Couldn't find all physical volumes for volume group VG.
...
You may be able to find the UUID for the physical volume that was overwritten by looking in the /etc/lvm/archive directory. Look in the file VolumeGroupName_xxxx.vg for the last known valid archived LVM metadata for that volume group.
Alternately, you may find that deactivating the volume and setting the partial (-P) argument will enable you to find the UUID of the missing corrupted physical volume.
[root@link-07 backup]# vgchange -an --partial
Partial mode. Incomplete volume groups will be activated read-only.
Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
...
Use the --uuid and --restorefile arguments of the pvcreate command to restore the physical volume. The following example labels the /dev/sdh1 device as a physical volume with the UUID indicated above, FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk. This command restores the physical volume label with the metadata information contained in VG_00050.vg, the most recent good archived metatdata for volume group . The restorefile argument instructs the pvcreate
command to make the new physical volume compatible with the old one
on the volume group, ensuring that the the new metadata will not be
placed where the old physical volume contained data (which could
happen, for example, if the original pvcreate
command had used the command line arguments that control metadata
placement, or it the physical volume was originally created using a
different version of the software that used different defaults). The pvcreate command overwrites only the LVM metadata areas and does not affect the existing data areas.
[root@link-07 backup]# pvcreate --uuid "FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk" --restorefile /etc/lvm/archive/VG_00050.vg /dev/sdh1
Physical volume "/dev/sdh1" successfully created
You can then use the vgcfgrestore command to restore the volume group's metadata.
[root@link-07 backup]# vgcfgrestore VG
Restored volume group VG
You can now display the logical volumes.
[root@link-07 backup]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
stripe VG -wi--- 300.00G /dev/sdh1 (0),/dev/sda1(0)
stripe VG -wi--- 300.00G /dev/sdh1 (34728),/dev/sdb1(0)
The following commands activate the volumes and display the active volumes.
[root@link-07 backup]# lvchange -ay /dev/VG/stripe
[root@link-07 backup]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Devices
stripe VG -wi-a- 300.00G /dev/sdh1 (0),/dev/sda1(0)
stripe VG -wi-a- 300.00G /dev/sdh1 (34728),/dev/sdb1(0)
If
the on-disk LVM metadata takes as least as much space as what overrode
it, this command can recover the physical volume. If what overrode the
metadata went past the metadata area, the data on the volume may have
been affected. You might be able to use the fsck command to recover that data.
e) Removing Lost Physical Volumes from a Volume Group :-
If you lose a physical volume, you can activate the remaining physical volumes in the volume group with the --partial argument of the vgchange command. You can remove all the logical volumes that used that physical volume from the volume group with the --removemissing argument of the vgreduce command.
It is recommended that you run the vgreduce command with the --test argument to verify what you will be destroying.
Like most LVM operations, the vgreduce command is reversible in a sense if you immediately use the vgcfgrestore command to restore the volume group metadata to its previous state. For example, if you used the --removemissing argument of the vgreduce command without the --test
argument and find you have removed logical volumes you wanted to
keep, you can still replace the physical volume and use another vgcfgrestore command to return the volume group to its previous state.
Recovering a Lost LVM Volume Disk
Server Configuration
In
all three examples, a server with SUSE Linux Enterprise Server 10 with
Service Pack 1 (SLES10 SP1) will be used with LVM version 2. The
examples will use a volume group called "sales" with a linear logical
volume called "reports". The logical volume and it's mount point are
shown below. You will need to substitute your mount points and volume
names as needed to match your specific environment.
ls-lvm:~ # cat /proc/partitions
major minor #blocks name
8 0 4194304 sda
8 1 514048 sda1
8 2 1052257 sda2
8 3 1 sda3
8 5 248976 sda5
8 16 524288 sdb
8 32 524288 sdc
8 48 524288 sdd
ls-lvm:~ # pvcreate /dev/sda5 /dev/sd[b-d]
Physical volume "/dev/sda5" successfully created
Physical volume "/dev/sdb" successfully created
Physical volume "/dev/sdc" successfully created
Physical volume "/dev/sdd" successfully created
ls-lvm:~ # vgcreate sales /dev/sda5 /dev/sd[b-d]
Volume group "sales" successfully created
ls-lvm:~ # lvcreate -n reports -L +1G sales
Logical volume "reports" created
ls-lvm:~ # pvscan
PV /dev/sda5 VG sales lvm2 [240.00 MB / 240.00 MB free]
PV /dev/sdb VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdc VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdd VG sales lvm2 [508.00 MB / 500.00 MB free]
Total: 4 [1.72 GB] / in use: 4 [1.72 GB] / in no VG: 0 [0 ]
ls-lvm:~ # vgs
VG #PV #LV #SN Attr VSize VFree
sales 4 1 0 wz--n- 1.72G 740.00M
ls-lvm:~ # lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
reports sales -wi-ao 1.00G
ls-lvm:~ # mount | grep sales
/dev/mapper/sales-reports on /sales/reports type ext3 (rw)
ls-lvm:~ # df -h /sales/reports
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/sales-reports
1008M 33M 925M 4% /sales/reports
Disk Belonging to a Volume Group Removed
Removing
a disk, belonging to a logical volume group, from the server may sound
a bit strange, but with Storage Area Networks (SAN) or fast paced
schedules, it happens.
Symptom:
The first thing you may notice when the server boots are messages like:
"Couldn't find all physical volumes for volume group sales."
"Couldn't find device with uuid '56pgEk-0zLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'."
'Volume group "sales" not found'
If
you are automatically mounting /dev/sales/reports, then the server
will fail to boot and prompt you to login as root to fix the problem.
Type root's password.
Edit the /etc/fstab file.
Comment out the line with /dev/sales/report
- Reboot
The LVM symptom is a missing sales volume group. Typing cat /proc/partitions confirms the server is missing one of it's disks.
ls-lvm:~ # cat /proc/partitions
major minor #blocks name
8 0 4194304 sda
8 1 514048 sda1
8 2 1052257 sda2
8 3 1 sda3
8 5 248976 sda5
8 16 524288 sdb
8 32 524288 sdc
ls-lvm:~ # pvscan
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
PV /dev/sda5 VG sales lvm2 [240.00 MB / 240.00 MB free]
PV /dev/sdb VG sales lvm2 [508.00 MB / 0 free]
PV unknown device VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdc VG sales lvm2 [508.00 MB / 500.00 MB free]
Total: 4 [1.72 GB] / in use: 4 [1.72 GB] / in no VG: 0 [0 ]
Solution:
Fortunately, the meta data and file system on the disk that was /dev/sdc are intact.
So the recovery is to just put the disk back.
Reboot the server.
The /etc/init.d/boot.lvm start script will scan and activate the volume group at boot time.
- Don't forget to uncomment the /dev/sales/reports device in the /etc/fstab file.
If this procedure does not work, then you may have corrupt LVM meta data.
Corrupted LVM Meta Data
The
LVM meta data does not get corrupted very often; but when it does, the
file system on the LVM logical volume should also be considered
unstable. The goal is to recover the LVM volume, and then check file
system integrity.
Symptom 1:
Attempting to activate the volume group gives the following:
ls-lvm:~ # vgchange -ay sales
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
Couldn't read volume group metadata.
Volume group sales metadata is inconsistent
Volume group for uuid not found: m4Cg2vkBVSGe1qSMNDf63v3fDHqN4uEkmWoTq5TpHpRQwmnAGD18r44OshLdHj05
0 logical volume(s) in volume group "sales" now active
This
symptom is the result of a minor change in the meta data. In fact,
only three bytes were overwritten. Since only a portion of the meta
data was damaged, LVM can compare it's internal check sum against the
meta data on the device and know it's wrong. There is enough meta data
for LVM to know that the "sales" volume group and devices exit, but
are unreadable.
ls-lvm:~ # pvscan
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
/dev/sdc: Checksum error
PV /dev/sda5 VG sales lvm2 [240.00 MB / 240.00 MB free]
PV /dev/sdb VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdc VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdd VG sales lvm2 [508.00 MB / 500.00 MB free]
Total: 4 [1.72 GB] / in use: 4 [1.72 GB] / in no VG: 0 [0 ]
Notice
pvscan shows all devices present and associated with the sales volume
group. It's not the device UUID that is not found, but the volume
group UUID.
Solution 1:
Since the disk was never removed, leave it as is.
There were no device UUID errors, so don't attempt to restore the UUIDs.
- This is a good candidate to just try restoring the LVM meta data.
ls-lvm:~ # vgcfgrestore sales
/dev/sdc: Checksum error
/dev/sdc: Checksum error
Restored volume group sales
ls-lvm:~ # vgchange -ay sales
1 logical volume(s) in volume group "sales" now active
ls-lvm:~ # pvscan
PV /dev/sda5 VG sales lvm2 [240.00 MB / 240.00 MB free]
PV /dev/sdb VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdc VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdd VG sales lvm2 [508.00 MB / 500.00 MB free]
Total: 4 [1.72 GB] / in use: 4 [1.72 GB] / in no VG: 0 [0 ]
- Run a file system check on /dev/sales/reports.
ls-lvm:~ # e2fsck /dev/sales/reports
e2fsck 1.38 (30-Jun-2005)
/dev/sales/reports: clean, 961/131072 files, 257431/262144 blocks
ls-lvm:~ # mount /dev/sales/reports /sales/reports/
ls-lvm:~ # df -h /sales/reports/
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/sales-reports
1008M 990M 0 100% /sales/reports
Symptom 2:
Minor damage to the LVM meta data is
easily fixed with vgcfgrestore. If the meta data is gone, or severely
damaged, then LVM will consider that disk as an "unknown device." If
the volume group contains only one disk, then the volume group and it's
logical volumes will simply be gone. In this case the symptom is the
same as if the disk was accidentally removed, with the exception of the
device name. Since /dev/sdc was not actually removed from the server,
the devices are still labeled a through d.
ls-lvm:~ # pvscan
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
PV /dev/sda5 VG sales lvm2 [240.00 MB / 240.00 MB free]
PV /dev/sdb VG sales lvm2 [508.00 MB / 0 free]
PV unknown device VG sales lvm2 [508.00 MB / 0 free]
PV /dev/sdd VG sales lvm2 [508.00 MB / 500.00 MB free]
Total: 4 [1.72 GB] / in use: 4 [1.72 GB] / in no VG: 0 [0 ]
Solution 2:
First, replace the disk. Most likely the disk is already there, just damaged.
- Since the UUID on /dev/sdc is not there, a vgcfgrestore will not work.
ls-lvm:~ # vgcfgrestore sales
Couldn't find device with uuid '56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu'.
Couldn't find all physical volumes for volume group sales.
Restore failed.
- Comparing
the output of cat /proc/partitions and pvscan shows the missing
device is /dev/sdc, and pvscan shows which UUID it needs for that
device. So, copy and paste the UUID that pvscan shows for /dev/sdc.
ls-lvm:~ # pvcreate --uuid 56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu /dev/sdc
Physical volume "/dev/sdc" successfully created
- Restore the LVM meta data
ls-lvm:~ # vgcfgrestore sales
Restored volume group sales
ls-lvm:~ # vgscan
Reading all physical volumes. This may take a while...
Found volume group "sales" using metadata type lvm2
ls-lvm:~ # vgchange -ay sales
1 logical volume(s) in volume group "sales" now active
- Run a file system check on /dev/sales/reports.
ls-lvm:~ # e2fsck /dev/sales/reports
e2fsck 1.38 (30-Jun-2005)
/dev/sales/reports: clean, 961/131072 files, 257431/262144 blocks
ls-lvm:~ # mount /dev/sales/reports /sales/reports/
ls-lvm:~ # df -h /sales/reports
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/sales-reports
1008M 990M 0 100% /sales/reports
Disk Permanently Removed
This
is the most severe case. Obviously if the disk is gone and
unrecoverable, the data on that disk is likewise unrecoverable. This is
a great time to feel good knowing you have a solid backup to rely on.
However, if the good feelings are gone, and there is no backup, how do
you recover as much data as possible from the remaining disks in the
volume group? No attempt will be made to address the data on the
unrecoverable disk; this topic will be left to the data recovery
experts.
Symptom:
The symptom will be the same as Symptom 2 in the Corrupted LVM Meta Data section above. You will see errors about an "unknown device" and missing device with UUID.
Solution:
Add a replacement disk to the server. Make sure the disk is empty.
- Create the LVM meta data on the new disk using the old disk's UUID that pvscan displays.
ls-lvm:~ # pvcreate --uuid 56ogEk-OzLS-cKBc-z9vJ-kP65-DUBI-hwZPSu /dev/sdc
Physical volume "/dev/sdc" successfully created
- Restore the backup copy of the LVM meta data for the sales volume group.
ls-lvm:~ # vgcfgrestore sales
Restored volume group sales
ls-lvm:~ # vgscan
Reading all physical volumes. This may take a while...
Found volume group "sales" using metadata type lvm2
ls-lvm:~ # vgchange -ay sales
1 logical volume(s) in volume group "sales" now active
- Run a file system check to rebuild the file system.
ls-lvm:~ # e2fsck -y /dev/sales/reports
e2fsck 1.38 (30-Jun-2005)
--snip--
Free inodes count wrong for group #5 (16258, counted=16384).
Fix? yes
Free inodes count wrong (130111, counted=130237).
Fix? yes
/dev/sales/reports: ***** FILE SYSTEM WAS MODIFIED *****
/dev/sales/reports: 835/131072 files (5.7% non-contiguous), 137213/262144 blocks
Mount the file system and recover as much data as possible.
- NOTE: If the missing disk contains
the beginning of the file system, then the file system's superblock
will be missing. You will need to rebuild or use an alternate
superblock. Restoring a file system superblock is outside the scope of
this article, please refer to your file system's documentation.
Conclusion
LVM
by default keeps backup copies of it's meta data for all LVM devices.
These backup files are stored in /etc/lvm/backup and /etc/lvm/archive.
If a disk is removed or the meta data gets damaged in some way, it can
be easily restored, if you have backups of the meta data. This is why
it is highly recommended to never turn off LVM's auto backup feature.
Even if a disk is permanently removed from the volume group, it can be
reconstructed, and often times the remaining data on the file system
recovered.
