Logical volume

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A logical volume is an abstraction of a disk partition provided by a volume manager. On Unix systems, it appears as a regular block device which can be used to hold filesystems or swap.

The volume group breaks up a backing store such as a disk partition into a pool of physical extents, and these extents are then re-assembled into logical volumes. In this way, logical volumes can suffer from internal fragmentation when the extents they are made up of are not contiguous on their underlying physical volumes. This can reduce I/O performance on slow-seeking magnetic media, such as hard disks, which have to then seek over the gaps between extents when doing large sequential reads or writes. Some volume managers attempt to minimize this by using large, fixed-size extents (a default of 4MB on the Linux LVM) in order to amortize the cost of these seeks. Other volume managers (such as Veritas) use variable-sized extents which are split only when necessary (to minimize the number of fragments in the common case).