Editing Logical volume management (section)

==Concepts==

=== Hybrid volume ===
A [[hybrid volume]] is any volume that intentionally and opaquely makes use of two separate physical volumes. 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&nbsp;X. [[ZFS]] also implements this functionality at the [[file system]] level, by allowing administrators to configure multi-level read/write caching.

Hybrid volumes present a similar concept as [[hybrid drive]]s, which also combine solid-state storage and rotational magnetic media.

===Snapshots===
Some volume managers also implement [[snapshot (computer storage)|snapshots]] by applying [[copy-on-write]] to each LE.  In this scheme, the volume manager will copy the LE to a ''copy-on-write table'' just before it is written to.  This preserves an old version of the LV, the snapshot, which may be later reconstructed by overlaying the copy-on-write table atop the current LV.  Unless the volume management supports both thin provisioning and discard, once an LE in the origin volume is written to, it is permanently stored in the snapshot volume.  If the snapshot volume was made smaller than its origin, which is a common practice, this may render the snapshot inoperable.

Snapshots can be useful for backing up self-consistent versions of volatile data such as table files from a busy database, or for rolling back large changes (such as an operating system upgrade) in a single operation.  Snapshots have a similar effect as rendering storage [[quiesce]]nt, and are similar to the [[shadow copy]] (VSS) service in Microsoft Windows. 

Some Linux-based [[Live CD]]s also use snapshots to simulate read-write access to a read-only [[optical disc]].