Logical volume

In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes. In particular, a volume manager can concatenate, stripe together or otherwise combine partitions (or block devices in general) into larger virtual ones that administrators can re-size or move, potentially without interrupting system use.

Volume management represents just one of many forms of storage virtualization; its implementation takes place in a layer in the device-driver stack of an operating system (OS) (as opposed to within storage devices or in a network).

Most volume-manager implementations share the same basic design. They start with physical volumes (PVs), which can be either hard disks, hard disk partitions, or Logical Unit Numbers (LUNs) of an external storage device. Volume management treats each PV as being composed of a sequence of chunks called physical extents (PEs). Some volume managers (such as that in HP-UX and Linux) have PEs of a uniform size; others (such as that in Veritas) have variably-sized PEs that can be split and merged at will.

Normally, PEs simply map one-to-one to logical extents (LEs). With mirroring, multiple PEs map to each LE. These PEs are drawn from a physical volume group (PVG), a set of same-sized PVs which act similarly to hard disks in a RAID1 array. PVGs are usually laid out so that they reside on different disks or data buses for maximum redundancy.

The system pools LEs into a volume group (VG). The pooled LEs can then be concatenated together into virtual disk partitions called logical volumes or LVs. Systems can use LVs as raw block devices just like disk partitions: creating mountable file systems on them, or using them as swap storage.

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