8.4. Thinly-provisioned snapshot volumes
Red Hat Enterprise Linux supports thinly-provisioned snapshot volumes. A snapshot of a thin logical volume also creates a thin logical volume (LV). A thin snapshot volume has the same characteristics as any other thin volume. You can independently activate the volume, extend the volume, rename the volume, remove the volume, and even snapshot the volume.
Similarly to all LVM snapshot volumes, and all thin volumes, thin snapshot volumes are not supported across the nodes in a cluster. The snapshot volume must be exclusively activated on only one cluster node.
Traditional snapshots must allocate new space for each snapshot created, where data is preserved as changes are made to the origin. But thin-provisioning snapshots share the same space with the origin. Snapshots of thin LVs are efficient because the data blocks common to a thin LV and any of its snapshots are shared. You can create snapshots of thin LVs or from the other thin snapshots. Blocks common to recursive snapshots are also shared in the thin pool.
Thin snapshot volumes provide the following benefits:
- Increasing the number of snapshots of the origin has a negligible impact on performance.
- A thin snapshot volume can reduce disk usage because only the new data is written and is not copied to each snapshot.
- There is no need to simultaneously activate the thin snapshot volume with the origin, which is a requirement of traditional snapshots.
- When restoring an origin from a snapshot, it is not required to merge the thin snapshot. You can remove the origin and instead use the snapshot. Traditional snapshots have a separate volume where they store changes that must be copied back, that is, merged to the origin to reset it.
- There is a significantly higher limit on the number of allowed snapshots as compared to the traditional snapshots.
Although there are many advantages for using thin snapshot volumes, there are some use cases for which the traditional LVM snapshot volume feature might be more appropriate to your needs. You can use traditional snapshots with all types of volumes. However, to use thin-snapshots requires you to use thin-provisioning.
You cannot limit the size of a thin snapshot volume; the snapshot uses all of the space in the thin pool, if necessary. In general, you should consider the specific requirements of your site when deciding which snapshot format to use.
By default, a thin snapshot volume is skipped during normal activation commands.
