Logical Volume Manager (204.3)

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

• Tools in the LVM suite

• Resizing, renaming, creating, and removing logical volumes, volume groups, and physical volumes

• Creating and maintaining snapshots

• Activating volume groups

Terms and Utilities

• /sbin/pv*

• /sbin/vg*

• /sbin/lv*

• mount

• /dev/mapper

• lvm.conf

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

• fdisk 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)

• 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:

Physical Extents (PE)

• Physical Extents 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)

• Logical Volume 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. mount 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.

lvcreate lvol 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

    # 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

    # 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 NDASH 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 mount

    # 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

    # 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_grow

    # 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

    # vgextend volume01 /dev/hda6

First device hda6 has to be converted into a physical volume with the command: pvcreate

    # pvcreate /dev/hda6

LVM Snapshots

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

    # lvcreate -L 50M -s -n snapshot0 /dev/volume01/lvol0

This will create a logical volume /dev/volume01/snapshot0, which then can be used, among others, 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

    # 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 - pvchange Change attributes of a physical volume

pvck - pvck Check physical volume metadata

pvcreate - pvcreate Initialize a disk or partition for use by LVM

pvdisplay - pvdisplay Display attributes of a physical volume

pvmove - pvmove Move physical extents

pvremove - pvremove Remove a physical volume

pvs - pvs Report information about physical volumes

pvscan - pvscan Scan all disk for physical volumes

lvchange - lvchange Change attributes for a logical volume

lvcreate - lvcreate Create a logical volume in an existing volume group

lvdisplay - lvdisplay Display attributes of a logical volumes

lvextend - lvextend Extend the size of a logical volume

lvmdiskscan - lvmdiskscan Scan for all devices visible to LVM2

lvreduce - lvmreduce Reduce the size of a logical volume

lvremove - lvmremove Remove a logical volume

lvrename - lvmrename Rename a logical volume

lvresize - lvmresize Resize a logical volume

lvs - lvs Report information about logical volumes

lvscan - lvscan Scan (all) disks for logical volumes

vgcfgbackup - vgcfgbackup Backup volume group descriptor area

vgchange - vgchange Change attributes of a volume group

vgck - vgck Check volume group metadata

vgconvert - vgconvert Convert volume group metadata

vgcreate - vgcreate Create a volume group

vgdisplay - vgdisplay Display attributes of volume groups

vgextend - vgextend Add physical volumes to a volume group

vgexport - vgexport Make volume groups unknown to the system

vgimport - vgimport Make exported volume groups known to the system

vgmerge - vgmerge Merge two volume groups

vgmknodes - vgmknodes Recreate volume group directory and logical volume special files

vgreduce - vgreduce Reduce a volume group

vgremove - vgremove Remove a volume group

vgrename - vgrename Rename a volume group

vgs - vgs Report information about volume groups

vgscan - vgscan Scan all disk for volume groups and rebuild caches

vgsplit - 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.

lvm.conf

At system startup lvm.conf configuration is loaded. The default directory is /etc/lvm. But it can be set with the environment variable LVM_SYSTEM_DIR. In the lvm.conf file you can specify additional configuration to load. To display the current settings, you can execute the lvm dumpconfig command.

vgscan pvs The vgscan command scans for block devices with lvm metadata on it. These physical volumes are stored in the lvm cache. This command uses the lvm.conf file to determine that. When for instance you use multipath devices, the metadata is the same on all the disks, on the different paths. Which will cause a lot of duplicates when you run pvs. Also when you want to boot of a multipath device, it can boot of a single path. You can use the filter option in the devices section for this in the lvm.conf, to skip the other devices and only look for multipath devices. The filter option uses regular expression to accept or reject block devices, like in below examples. Add all discovered devices:

            filter = [ "a/.*/" ]

Filter to remove cdrom devices:

            filter = [ "r|/dev/cdrom|" ]

Filter only device mapper names on a multipath device:

            filter = [ "a|/dev/disk/by-id/dm-uuid-.*-mpath-.*|", "r|.*|" ]

This filter just adds partition 8 on the first IDE Harddrive and removes all other block devices.

            filter = [ "a|^/dev/hda8$|", "r/.*/" ]

Other usefull and commonly used option in the devices sections in the lvm.conf are: preferred_names The earliest pattern is used in any output like pvs. So in our example of multipath devices we want.

            preferred_names = [ "^/dev/mpath/", "^/dev/mapper/mpath", "^/dev/[hs]d" ]

types If you boot from a local device and the rest are multipath devices, you need to change the types option, next to the filter option with this example. I tells LVM the acceptable block devices (device-mapper) and the allowed partitions (253)

            types = [ "device-mapper", 253 ]

For other sections with options check: man lvm.conf