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Logical Volume


LVM Architecture Overview

For the Red Hat Enterprise Linux 4 release of the Linux operating system, the original LVM1 logical volume manager was replaced by LVM2, which has a more generic kernel framework than LVM1. LVM2 provides the following improvements over LVM1:
  • flexible capacity
  • more efficient metadata storage
  • better recovery format
  • new ASCII metadata format
  • atomic changes to metadata
  • redundant copies of metadata
LVM2 is backwards compatible with LVM1, with the exception of snapshot and cluster support. You can convert a volume group from LVM1 format to LVM2 format with the vgconvert Command. For information on converting LVM metadata format, see the vgconvert(8) man page.
The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. This device is initialized as an LVM physical volume(PV).
To create an LVM logical volume, the physical volumes are combined into a volume group (VG). This creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated. This process is analogous to the way in which disks are divided into partitions. A logical volume is used by file systems and applications (such as databases).
Figure 1.1, “LVM Logical Volume Components” shows the components of a simple LVM logical volume:
LVM Logical Volume Components
LVM Logical Volume Components

Basic LVM commands

Initializing disks or disk partitions

To use LVM, partitions and whole disks must first be converted into physical volumes (PVs) using the pvcreate command. For example, to convert /dev/hda and /dev/hdb into PVs use the following commands:

pvcreate /dev/hda
pvcreate /dev/hdb
If a Linux partition is to be converted make sure that it is given partition type 0x8E usingfdisk, then use pvcreate:

pvcreate /dev/hda1

Creating a volume group

Once you have one or more physical volumes created, you can create a volume group from these PVs using the vgcreate command. The following command:

vgcreate  volume_group_one /dev/hda /dev/hdb
creates a new VG called volume_group_one with two disks, /dev/hda and /dev/hdb, and 4 MB PEs. If both /dev/hda and /dev/hdb are 128 GB in size, then the VGvolume_group_one will have a total of 2**16 physical extents that can be allocated to logical volumes.
Additional PVs can be added to this volume group using the vgextend command. The following commands convert /dev/hdc into a PV and then adds that PV tovolume_group_one:

pvcreate /dev/hdc
vgextend volume_group_one /dev/hdc
This same PV can be removed from volume_group_one by the vgreduce command:

vgreduce volume_group_one /dev/hdc
Note that any logical volumes using physical extents from PV /dev/hdc will be removed as well. This raises the issue of how we create an LV within a volume group in the first place.

Creating a logical volume

We use the lvcreate command to create a new logical volume using the free physical extents in the VG pool. Continuing our example using VG volume_group_one (with two PVs /dev/hda and /dev/hdb and a total capacity of 256 GB), we could allocate nearly all the PEs in the volume group to a single linear LV called logical_volume_one with the following LVM command:

lvcreate -n logical_volume_one   --size 255G volume_group_one
Instead of specifying the LV size in GB we could also specify it in terms of logical extents. First we use vgdisplay to determine the number of PEs in the volume_group_one:

vgdisplay volume_group_one | grep "Total PE"
which returns

Total PE   65536
Then the following lvcreate command will create a logical volume with 65536 logical extents and fill the volume group completely:

lvcreate -n logical_volume_one  -l 65536 volume_group_one
To create a 1500MB linear LV named logical_volume_one and its block device special file /dev/volume_group_one/logical_volume_one use the following command:

lvcreate -L1500 -n logical_volume_one volume_group_one
The lvcreate command uses linear mappings by default.
Striped mappings can also be created with lvcreate. For example, to create a 255 GB large logical volume with two stripes and stripe size of 4 KB the following command can be used:

lvcreate -i2 -I4 --size 255G -n logical_volume_one_striped volume_group_one
It is possible to allocate a logical volume from a specific physical volume in the VG by specifying the PV or PVs at the end of the lvcreate command. If you want the logical volume to be allocated from a specific physical volume in the volume group, specify the PV or PVs at the end of the lvcreate command line. For example, this command:

lvcreate -i2 -I4 -L128G -n logical_volume_one_striped volume_group_one /dev/hda /dev/hdb
creates a striped LV named logical_volume_one that is striped across two PVs (/dev/hda and /dev/hdb) with stripe size 4 KB and 128 GB in size.
An LV can be removed from a VG through the lvremove command, but first the LV must be unmounted:

umount /dev/volume_group_one/logical_volume_one
lvremove /dev/volume_group_one/logical_volume_one
Note that 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/<volume_group_name>/<logical_volume_name>
so that if we had two volume groups myvg1 and myvg2 and each with three logical volumes named lv01, lv02, lv03, six device special files would be created:

/dev/myvg1/lv01
/dev/myvg1/lv02
/dev/myvg1/lv03
/dev/myvg2/lv01
/dev/myvg2/lv02
/dev/myvg2/lv03

Extending a logical volume

An LV can be extended by using the lvextend command. You can specify either an absolute size for the extended LV or how much additional storage you want to add to the LVM. For example:

lvextend -L120G /dev/myvg/homevol
will extend LV /dev/myvg/homevol to 12 GB, while

lvextend -L+10G /dev/myvg/homevol
will extend LV /dev/myvg/homevol by an additional 10 GB. Once a logical volume has been extended, the underlying file system can be expanded to exploit the additional storage now available on the LV. With Red Hat Enterprise Linux 4, it is possible to expand both the ext3fs and GFS file systems online, without bringing the system down. (The ext3 file system can be shrunk or expanded offline using the ext2resize command.) To resize ext3fs, the following command

ext2online /dev/myvg/homevol
will extend the ext3 file system to completely fill the LV, /dev/myvg/homevol, on which it resides.
The file system specified by device (partition, loop device, or logical volume) or mount point must currently be mounted, and it will be enlarged to fill the device, by default. If an optional size parameter is specified, then this size will be used instead.