Release Notes

30 Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines | BP1027
7 Planning and design best practices
7.1 Use ASM for Windows and VSM for VMware
To ensure that consistent snapshot and clones are created for Windows, ASM/ME must be used so that VSS
is invoked before the snapshot is performed by the underlying hardware (the storage arrays). When a clone
volume is created, ASM will call the VSS writer to freeze I/O and flush any unwritten data prior to creating the
clone volume, resulting in an application consistent clone.
To ensure that hypervisor aware snapshots and clones are created in VMware environments, Dell Virtual
Storage Manager (VSM) for VMware® should be used. This allows integration with the VMware client utilities
and Virtual Center so that snapshot and clone operations are passed to the hardware (the PS Series group)
to perform where applicable.
7.2 Snapshot reserve
Snapshot reserve defaults to 100% of the base volume capacity. This is to ensure that a single snapshot can
be held even if 100% of the base volume is modified, but it can be changed at any time without taking the
volume offline. Snapshot reserve can be increased to values greater than 100% (up to 10,000%) or it can be
completely disabled by setting the value to 0%. The global defaults may also be adjusted by going to the
Defaults tab under Group Configuration (Figure 13). To conserve free pool space, snapshot reserve can
always be disabled for a volume if there are no plans to use snapshots, or if another method (such as an
application-level snapshot or replication) is being used instead.
Default volume settings
Administrators should use the Dell EqualLogic Group Manager or SAN HQ to monitor the usage of snapshot
reserve space to ensure that there is adequate space to retain the desired number of snapshots. The number
of snapshots retained should be driven by the organizations RTO/RPO (Recovery Time Object/Recovery
Point Object). For example, if a business determines that an acceptable loss of data is one hour, then
administrators may schedule a snapshot at least once every hour during business hours.
The rate that data changes on the base volume will determine how much snapshot reserve is consumed
between snapshots. If multiple snapshots are retained, then multiply the estimated amount of snapshot
reserve needed by the number of snapshots retained.