Revision: $Revision: 910 $ ($Date: 2012-05-24 15:46:43 +0200 (Thu, 24 May 2012) $)
Candidates should be able to create and remove logical volumes, volume groups, and physical volumes. This objective includes snapshots and resizing logical volumes.
Key Knowledge Areas:
The following is a partial list of used files, terms, and utilities:
lvm is a logical volume manager for Linux. It enables you to concatenate several physical volumes (hard disks etc.) to so-called volume groups, forming a storage pool, much like a virtual disk. IDE and SCSI disks, as well as multiple devices (MD) are supported.
In the figure below, the concepts and terminology used by lvm are sketched. On the right side the names of the commands are shown that can be used to create and/or manipulate the layer sketched on the left.
a hard disk, or a partition, e.g.
You should set the partition types of the disk or partition to
0x8e, which is “Linux LVM”.
Partitioning is done using fdisk. Please note that
your version of fdisk may not yet know this type,
so it will be listed as “Unknown”. You can turn any consecutive number
of blocks on a block device into a Physical Volume:
a physical medium with some administrative data added to it. The command pvcreate can be used to add the administration onto the physical medium. The command vgcreate is used to create a volume group, which consists of one or more PV's. A PV that has been grouped in a volume group contains Physical Extents:
To the linux kernel, there is no difference between a regular partition and a Logical
Volume. A simple mount suffices to be able to use your logical
Some examples of typical usage of the LVM commandset follow. Initially, you need
to set the partition type for the partitions to use to create logical volumes to
0x8e. Let's assume we have partitions
/dev/hda5, and they are set
to the correct partitioning type. To create a physical volume on both
partitions (i.e. to set up the volume group descriptor) you type (being
# pvcreate /dev/hda4 /dev/hda5
Now we have created two physical volumes. Next, we will create a volume group.
A volume group needs to have a name (we choose
To create our volume group, using our previously defined physical volumes, type:
# vgcreate volume01 /dev/hda5 /dev/hda4
The previous command line is the most basic form (refer to the manual pages for a list of configurable parameters). This will create an array of physical extents, by default they are 4 Mb in size. Using these extents we can create one or more logical volumes, e.g:
# lvcreate -L 100M volume01
.. this creates a logical volume with a default name choosen by
lvcreate and starts with the string
lvol followed by a digit
-- let's assume
lvol0. The logical volume will be
created using the volumegroup
volume01. The name of
the devicefile for this volume will be
Next, we can make a filesystem on the volumegroup, as usual,
using mkfs, e.g. an
# mkfs -t xfs /dev/volume01/lvol0
The resulting filesystem can be mounted as usual:
# mount /dev/volume01/lvol0 /mnt
A logical volume can be modified in order to create more space for the filesystem that is on top of this logical volume. Assuming that there is enough space in the volume group a logical volume can be increased in size with the command:
# lvextend -L +50M /dev/volume01/lvol0
After the logical volume has been increased the filesystem on top of the logical volume still has the same size. To use the extra space of the logical volume the filesystem needs to be resized. This can be done by the command:
# xfs_growfs /dev/volume01/lvol0
For an ext2/ext3/ext4 file system use the command resize2fs. Note that for resizing the filesystem it must not be mounted.
In the previous example it was assumed that there was enough free space in the volume group. If this is not the case, extra disk space can be added to the volume group in a similar way. To do so use the command:
# vgextend volume01 /dev/hda6
First device hda6 has to be converted into a physical volume with the command:
# pvcreate /dev/hda6
One of the nicest features of LVM is the possibility of taking snapshots of volumes. A snapshot is a virtual copy of the volume to enable easy backups. LVM snapshots use a strategy called "copy on write". This means that the snapshot logical volume only saves data blocks from the original logical volume that are changed in the original logical volume. To do so the logical volume manager first reads the (unchanged) data block on the original and than writes the data block to the snapshot. On filesystems with many changes (e.g. databases) this can lead to performance issues.
The -s option in the lvcreate command specifies that the newly created logical volume is a snapshot.
# lvcreate -L 50M -s -n snapshot0 /dev/volume01/lvol0
This will create a logical volume
which then can be used f.i. (, for instance, ?) for backup purposes. The advantage of
the snapshot is that
the data is consistent, i.e. the data doesn't change during backup.
After the backup is finished, the snapshot has to be removed otherwise the performance issues mentioned earlier will start to come into play. To remove the snapshot use:
This section gives an overview of most lvm related commands. The manual pages describe the commands more in detail.
Change attributes of a physical volume
Check physical volume metadata
Initialize a disk or partition for use by LVM
Display attributes of a physical volume
Move physical extents
Remove a physical volume
Report information about physical volumes
Scan all disk for physical volumes
Change attributes for a logical volume
Create a logical volume in an existing volume group
Display attributes of a logical volumes
Extend the size of a logical volume
Scan for all devices visible to LVM2
Reduce the size of a logical volume
Remove a logical volume
Rename a logical volume
Resize a logical volume
Report information about logical volumes
Scan (all) disks for logical volumes
Backup volume group descriptor area
Change attributes of a volume group
Check volume group metadata
Convert volume group metadata
Create a volume group
Display attributes of volume groups
Add physical volumes to a volume group
Make volume groups unknown to the system
Make exported volume groups known to the system
Merge two volume groups
Recreate volume group directory and logical volume special files
Reduce a volume group
Remove a volume group
Rename a volume group
Report information about volume groups
Scan all disk for volume groups and rebuild caches
Split a volume group into two
The device mapper is a kernel driver that provides a framework for volume management. It provides a generic way of creating mapped devices, which may be used as logical volumes. It does not specifically know about volume groups or metadata formats.
LVM logical volumes are activated using the device mapper. Each logical volume is translated into a mapped device. Each segment translates into a line in the mapping table that describes the device. The device mapper supports a variety of mapping targets, including linear mapping, striped mapping, and error mapping.