Administrator Guide

• If you did not enable the option to borrow free pool space (or if you enabled the option, but not enough free pool space is
available), the primary group deletes the failback snapshot and generates an event message. To reestablish the failback snapshot,
increase the local replication reserve and replicate the volume.
In addition, if you attempt to replicate a recovery volume to the primary group, and not enough local replication reserve is available to
store the changes, the primary group generates an event message advising you to increase the space.
Guidelines for 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 percent ensures sucient local replication reserve even if the entire volume changes during a replication
operation.
• Space-ecient value
If few volume changes are expected during an average replication operation, use a value less than 100 percent.
To obtain an appropriate value, estimate the average volume change that occurs during a typical replication operation. Then, use the
following equation, where 5 percent is the minimum local replication reserve:
5 percent + change rate
For example, if you estimate that at most 10 percent of the volume changes, a value of 15 percent might be appropriate (5 percent
plus 10 percent).
If you use a local replication reserve value that is less than 100 percent, sucient 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 not enough local replication reserve is available
to complete a replication.
Guidelines for 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 percent ensures sucient local replication reserve even if the entire volume changes between replication
operations.
• Space-ecient value
If, on average, few volume changes are expected between replication operations, use a value less than 100 percent.
To obtain an appropriate value, estimate the average volume change that occurs between consecutive replication operations. Then,
use the following equation, where 5 percent is the minimum local replication reserve:
5 percent + change rate
For example, if you estimate that at most 20 percent of the volume changes, a value of 25 percent might be appropriate (5 percent
plus 20 percent).
If you use a local replication reserve value that is less than 100 percent, sucient space might not be available to maintain the
failback snapshot. Therefore, Dell recommends that you select the option that allows you to borrow free pool space if not enough
local replication reserve is available.
Guidelines for Sizing Local Replication Reserve
On the primary group, you can specify the value of the local replication reserve and whether to keep the failback snapshot when you
congure a volume for replication. You can also enable the option that allows you to borrow free pool space if not enough local
replication reserve is available. You can later modify these settings as needed.
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