LVM

LVM advantages:

Through these different types of logical-to-physical mappings, LVM can achieve four important advantages over raw physical partitions:

1. Logical volumes can be resized while they are mounted and accessible by the database or file system, removing the downtime associated with adding or deleting storage from a Linux server
2. Data from one (potentially faulty or damaged) physical device may be relocated to another device that is newer, faster or more resilient, while the original volume remains online and accessible
3. Logical volumes can be constructed by aggregating physical devices to increase performance (via disk striping) or redundancy (via disk mirroring and I/O multipathing)
4. Logical volume snapshots can be created to represent the exact state of the volume at a certain point-in-time, allowing accurate backups to proceed simultaneously with regular system operation

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 using fdisk, 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 VG volume_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 to volume_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

After using lvextend command the file system also to be resized using below (umount is not neccessary)

   # resize2fs /dev/myvg/homevol

After using lvreduce command the file system also to be resized using below (umount is not neccessary)

#umount the lvm

# lvm vgchange -a y
# e2fsck -f /dev/VolGroup00/LogVol00
# resize2fs -f /dev/VolGroup00/LogVol00 10G
# lvreduce -L10G /dev/VolGroup00/LogVol00

(LVM size will be 10G)