LVM Clustering

The Linux Logical Volume Manager (LVM) manages storage space, allowing storage drive space to be added or expanded on the fly without system reboots or shutdowns, giving increased flexibility for operations

Among LVMs features are two related to High Availability

The first is that a snap shot of the disk can be taken at a particular time, allowing a backup to be taken while the application runs without the two interferring with each other. In this scenario, the application would be stopped during a quiet time, a snap shot created and the application restarted - the whole process should take a handful of seconds, and go virtually unnoticed, with the benefit that the snap shot contains a complete copy of the data at the time of taking it without any files being partly modified on the backup copy.

LVM accomplishes this by undertaking a copy-on-write transaction when data in the snapshot is updated by the system, this allows the snapshot to be created "cheaply" without requiring significant I/O, time or disk space.

The second is that LVM allows you to tailor the amount of space allocated to a file system dynamically, removing the need for cludges (e.g. symbolic links pointing to a new disk), reducing administration and the requirement for downtime to resize a filesystem.

LVM puts the physical volumes into storage pools called volume groups. An LVM can manage whole SCSI or IDE disks and partitions in a volume group, as well as hardware and even software RAID devices. A volume group is the equivalent of a physical disk from the system’s point of view. The equivalent of partitions into which the storage space is divided is called a logical volume.

LifeKeeper can support LVM volumes, through use of an add on ARK, this allows any of the other recovery kits to operate on top of Linux logical volumes. The LVM ARK adds in support for two new resource types, namely lvmlv and lvmvg for logical volumes and volume groups respectively.

The LVM ARK also provides support for Raw I/O on a LVM device, as well as allowing entire disks (e.g. /dev/sdc c.w. /dev/sdc1) to be used.

It should be noted, that at the current time, LifeKeeper does not support LVM2 which is a recent release of LVM by Sistina. It is anticipated that this restriction will be removed in the future.

An example of a resource hierarchy tree within LifeKeeper using LVM is as follows :

Original Source: www.openminds.co.uk