Release Notes

12 Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines | BP1027
3.4 Storage requirements and configuration limits
In a PS Series group, snapshots can be used to protect data against human error, viruses, or database
corruption. PS Series storage administrators can create a point-in-time copy or multiple copies of the base
volume, which can be retained for data protection or made accessible to another host. Creating snapshots
can be performed while the base volume remains online, therefore, users and applications are not disrupted.
Before creating a snapshot of a volume, the administrator must allocate snapshot reserve to hold the
snapshot data. Snapshot reserve space is always consumed from the same storage pool as the volume. By
having the administrator decide how much space to reserve, the storage keeps track of the commitment. This
allows the administrator to avoid having to manually estimate the free space that may be quickly consumed.
The PS Series Group snapshot reserve space, allocated by default, is equal to 100% of the host volume
allocated space. This ensures that a 100% data change in the volume can be protected by a single snapshot.
This value can, and often is, easily set to a lower value based on the application data change rate,
recoverability requirements, or role that the snapshot will be used for.
The amount of required snapshot reserve space depends on:
• The number of writes that occur to the base volume during the life of the snapshot. In general, initial
changes to the base volume will result in the utilization of more snapshot reserve space, while
subsequent writes to logical blocks that have already been modified do not require additional space.
• The range of logical blocks where the data change occurs. If the same blocks of data are consistently
modified, then snapshot reserve is consumed only when the first change is made.
• The number of simultaneous snapshots in use. More snapshots tend to use more space since
multiple points in time must be preserved.
• The life span of the snapshots. Longer-lived snapshots tend to use more space since more data is
likely to change over a longer time period.
Even though snapshot reserve space is not allocated until needed, it is immediately subtracted (or reserved)
from the pool and reflected in the available free space. This guarantees that the snapshot reserve space will
be available to hold any new updates to the volume as well as preserve the point-in-time view of the volume.
In addition, the system will automatically delete older snapshots as necessary to stay within the reserve
usage limit set by the administrator. This method reflects a more accurate representation of space that is
available for user data. An administrator can adjust or change a volume snapshot reserve at any time through
the GUI or CLI management utilities.