Logical Volume Manager (204.3)

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:

Resources: Impson00; Hubert00; Colligan00; the man pages for the various commands.

Configuring Logical Volume Management

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.

The physical media / partitions

a hard disk, or a partition, e.g. /dev/hda, /dev/hda6 or /dev/sda. 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:

Physical Volume (PV)

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:

Physical Extents (PE)

Physical Extents are blocks of diskspace, often several megabytes in size. Using the command lvcreate you can assign PEs to a Logical Volume:

Logical Volume (LV)

A Logical Volume. On a logical volume we can use the command mkfs to get a Filesystem:

Filesystem

ext2, ReiserFS, NWFS, XFS, JFX, NTFS etc. 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 volume.

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/hda4 and /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 the superuser):

# 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 volume01). 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 /dev/volume01/lvol0. Next, we can make a filesystem on the volumegroup, as usual, using mkfs, e.g. an xfs filesystem:

# mkfs -t xfs /dev/volume01/lvol0

The resulting filesystem can be mounted as usual:

# mount /dev/volume01/lvol0 /mnt

Modifying logical volumes, volume groups and physical volumes

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
   	  

LVM Snapshots

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 /dev/volume01/snapshot0, 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:

# lvremove /dev/volume01/snapshot0
      

LVM commands

This section gives an overview of most lvm related commands. The manual pages describe the commands more in detail.

pvchange

Change attributes of a physical volume

pvck

Check physical volume metadata

pvcreate

Initialize a disk or partition for use by LVM

pvdisplay

Display attributes of a physical volume

pvmove

Move physical extents

pvremove

Remove a physical volume

pvs

Report information about physical volumes

pvscan

Scan all disk for physical volumes

lvchange

Change attributes for a logical volume

lvcreate

Create a logical volume in an existing volume group

lvdisplay

Display attributes of a logical volumes

lvextend

Extend the size of a logical volume

lvmdiskscan

Scan for all devices visible to LVM2

lvreduce

Reduce the size of a logical volume

lvremove

Remove a logical volume

lvrename

Rename a logical volume

lvresize

Resize a logical volume

lvs

Report information about logical volumes

lvscan

Scan (all) disks for logical volumes

vgcfgbackup

Backup volume group descriptor area

vgchange

Change attributes of a volume group

vgck

Check volume group metadata

vgconvert

Convert volume group metadata

vgcreate

Create a volume group

vgdisplay

Display attributes of volume groups

vgextend

Add physical volumes to a volume group

vgexport

Make volume groups unknown to the system

vgimport

Make exported volume groups known to the system

vgmerge

Merge two volume groups

vgmknodes

Recreate volume group directory and logical volume special files

vgreduce

Reduce a volume group

vgremove

Remove a volume group

vgrename

Rename a volume group

vgs

Report information about volume groups

vgscan

Scan all disk for volume groups and rebuild caches

vgsplit

Split a volume group into two

Device mapper

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.

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