Release Notes

5 Dell PS Series: Space Borrowing for Snapshots and Replicas | TR1084 | version 2
1 Space borrowing
In a Dell PS Series group, snapshots and replicas are space-efficient, point-in-time copies of a volume that can be used
to protect data and are often used to recover data lost by human error and data corruption.
Snapshots and replicas require available disk space to operate successfully. This disk space is referred to as snapshot
reserve and replica reserve. To determine the optimal amount of reserve space for snapshots and replicas, it is a best
practice to use the default space values, monitor activity over time, analyze the space usage, and make adjustments as
necessary. These precautions will help you keep the desired number of snapshots and replicas while using space
efficiently.
Space usage is not always predictable, however, and sometimes it is difficult to determine the optimal values to use for
reserves. For example, if reserves are set too high, then space is wasted. If reserves are set too low, then it is difficult to
maintain the desired number of snapshots and replicas to keep. If, for example, free reserve space is low in a pool for
snapshots, or within delegated space for replicas, then creating another snapshot or replica can potentially force older
snapshots and replicas to be prematurely deleted.
Dell PS Series firmware version 8.0 provides the ability for snapshots, local and remote replicas, and deleted volumes in
the Group Volume Recovery Bin to temporarily borrow space beyond the configured reserves. This feature, called
space borrowing, simplifies configuring reserve space, improves space utilization, and enhances management of
snapshots and replica reserves.
Note: Previous versions of Dell PS Series firmware (6.x and 7.x) provided space borrowing for snapshots and local
replication operations only.
Borrowed space is a subset of the storage pool’s total space and can allow older snapshots and replicas to remain if
space is available to borrow. Borrowed space is automatically reclaimed when necessary. Space is potentially available
for borrowing from the following areas:
• Unused snapshot reserve
• Unused local replication reserve on the primary and secondary groups
• Unused delegated space
• Free space in the storage pool where the snapshot or replica resides; space cannot be borrowed from another
pool.
Space cannot be borrowed from:
• Unused volume reserve: Unused Space that has been reserved for volumes.
• NAS storage container: NAS containers are used by Dell Storage FS Series nodes to create NAS shares. Since these
volumes do not contain PS Series snapshots or replicas, they are not available for space borrowing.
• VMware® vSphere® Virtual volumes™ (vVols) storage container: vVol storage containers are used with VMware
environments and cannot be used to hold standard PS Series volumes, snapshots, replicas, or NAS containers.
Storage containers are not supported outside of the VMware Virtual Volumes context.
For examples on using Space Borrowing with Snapshots and Replicas, please refer to: Space Borrowing Examples.