Logical volume management

Volume management treats each PV as being composed of a sequence of chunks called physical extents (PEs).

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.

The pooled LEs can then be concatenated together into virtual disk partitions called logical volumes or LVs.

Most volume managers can perform this movement online; if the underlying hardware is hot-pluggable this allows engineers to upgrade or replace storage without system downtime.

For instance, a workload may consist of random seeks so an SSD may be used to permanently store frequently used or recently written data, while using higher-capacity rotational magnetic media for long-term storage of rarely needed data.

On Linux, bcache or dm-cache may be used for this purpose, while Fusion Drive may be used on OS X. ZFS also implements this functionality at the file system level, by allowing administrators to configure multi-level read/write caching.

Some Linux-based Live CDs also use snapshots to simulate read-write access to a read-only optical disc.

Logical volumes can suffer from external fragmentation when the underlying storage devices do not allocate their PEs contiguously.

With implementations that are solely volume management, such as Core Storage and Linux LVM, separating and abstracting away volume management from the file system loses the ability to easily make storage decisions for particular files or directories.