HP B6960-96035 manual Snapshots without the preallocation of disk space, Snapclones

not written to that reserved space until necessary. As the data changes on the source volume, the snapshot data on the target volume is updated with the original data.

Since this snapshot technique caches only the difference between the ever-changing original data content against the point-in-time state, copy-on-write snapshots with the preallocation of disk space are depended on their source volumes; if the data on source volumes is lost, the associated snapshots are useless.

Snapshots without the preallocation of disk space

Copy-on-write snapshots without the preallocation of disk space also represents a point-in-time copy of the original data but it does not require preallocation of disk capacity. The disk capacity is allocated dynamically on as-needed basis. As the data on source volume changes, free space in a disk array is used for the creation of the snapshot. Copy-on-write snapshots without the preallocation of disk space are intended to be short-lived snapshots. Note that their size grows dynamically and may eventually run out of storage capacity if they are not deleted regularly.

The main benefit of copy-on-write snapshots without the preallocation of disk space over copy-on-write snapshots with the preallocation of disk space is in significant reduction of costs. Considerably less additional storage capacity for replication space is needed, if the snapshots are deleted regularly, than with a standard snapshot technology.

Since this snapshot technique caches only the difference between the ever-changing original data content against the point-in-time state, copy-on-write snapshots without the preallocation of disk space are depended on their source volumes; if the data on source volumes is lost, the associated snapshots are useless.

Snapclones

The first part of the snapclone creation is similar to the creation of a copy-on-write snapshot with the preallocation of disk space, which is followed by the cloning process. During this process, all data from the source volume is copied to the target volume. A snapclone enables immediate access to the replicated data while the cloning process runs in the background using the disk array idle time. When the cloning process is finished, the snapclone becomes a full data copy that represents a point-in-time state of the source volume; if the data on source volume is lost, you can always revert to the snapclone.

280 Snapshot concepts