Dell EqualLogic PS6210XS EqualLogic Group Manager Administrator s Guide PS Ser - Page 242

• 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 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 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, sufficient 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 sufficient local replication reserve even if the entire volume changes between replication operations. • Space-efficient 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, sufficient 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 configure 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. 242 About Backing Up and Protecting Your Data