Hardware manual

Group Administration Volume replication
12–10
However, using the recommended values might not be the most efficient use of local replication reserve. Ideally,
you want to allocate only enough space to meet the volume requirements. However, specifying too little space can
prevent successful replication.
The optimal value for local replication reserve depends on the volume change rate
, the replication frequency, and
whether you are keeping the failback snapshot. The volume change rate can be difficult to estimate. See How
volume changes affect re
plication space on page 12-6.
If you want to use less than the recommende
d value for local replication reserve, follow these guidelines:
• No failback snapshot. Size the local replication reserve based
only on its use during replication.
• Keep the failback snapshot. Size the local replication reserve based on its
use during replication and also for
maintaining the failback snapshot. Then, combine the two values.
See Sizing local replication reserve for use
during replication and Sizing the local replication reserve for the
failback snapshot.
Sizing local replication reserve for use during replication
To size the portion of local replication reserve for use during replication, follow these guidelines:
• Recommended value. A
value of 100% ensures sufficient local replication reserve even if the entire volume
changes during a replication operation.
• Space-efficient value.
If few volume changes are expected during an average replication operation, use a
value less than 100%.
To obtain an appropriate value, estimate the average volume changes that occur durin
g a typical replication
operation. Then, use this equation, where 5% is the minimum local replication reserve:
5% + change rate
For example, if you estimate that at most 10% of the volume changes,
a value of 15% might be appropriate
(5% plus 10%).
If you use a local replication reserve value that is less than 100%, suf
ficient space might not be available to
complete a replication operation (for example, if more volume changes than expected occur during a
replication operation). Therefore, Dell recommends that you select the option that allows you to borrow free
pool space if there is not enough local replication reserve to complete a replication.
Sizing the local replication reserve for the failback snapshot
To size the portion of local replication reserve used to maintain the failback snapshot, follow these guidelines:
• Recommended value. A
value of 100% ensures sufficient local replication reserve even if the entire volume
changes between replication operations.
• Space-efficient value. If few volume
changes are expected on average between replication operations, use a
value less than 100%,
To obtain an appropriate value, estimate the average volume changes that occur betw
een consecutive
replication operations. Then, use this equation, where 5% is the minimum local replication reserve: