Release Notes

10 Dell PS Series: Space Borrowing for Snapshots and Replicas | TR1084 | version 2
2 Best practices for space borrowing
For a system to gain the optimum benefit of space borrowing, apply the following best practices when planning and
configuring snapshot and replication storage space.
2.1 Planning space requirements
Space borrowing is intended to help during peaks of activity when additional space is needed temporarily. It does not
take the place of carefully provisioning reserves. For additional information on planning space requirements for
snapshots and replicas, refer to the following documents:
• Dell EqualLogic Group Manager Administrator’s Manual on eqlsupport.dell.com (login required)
• Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines
• Using Dell PS Series Asynchronous Replication
2.2 Space borrowing prioritization
When necessary, the system automatically deletes a snapshot or replica that is borrowing space. Objects using
borrowed space are at risk for being deleted if the borrowed space is required to support another operation with a
higher priority. This could occur if a new snapshot of Volume A borrows snapshot reserve from Volume B and then
Volume B needs that space for its own snapshots.
Another example is space borrowed from free space in the storage pool that becomes needed to support the creation
of a new volume or expansion of an existing volume. This scenario could cause existing snapshots and replicas that are
borrowing the free space to be deleted. However, in-progress or last successful replicas are never deleted
automatically by this process.
2.3 Space borrowing behavior with changes to storage pools
If a volume or replica set is moved to another pool, the associated snapshots and replicas are bound to the space
borrowing limitations of the new pool. Also, if the size of a storage pool decreases due to the removal of a member,
this lowers the pool capacity as well as impacts the space borrowing capabilities.
2.4 Space borrowing for replication
While it is possible to enable or disable space borrowing for snapshots, space borrowing for replication is automatic
and cannot be disabled.
Remote replicas can borrow beyond their total replica reserve, but the total amount of configured reserve space must
still fit within the delegated space. If there is insufficient delegated space on the secondary group, the system requires
manual administrative intervention to increase the amount of delegated space.