Dell EqualLogic PS6210XS EqualLogic Group Manager Administrator s Guide PS Ser - Page 246

Guidelines for Sizing Replica Reserve for a Volume To determine the amount of space that the secondary group must delegate to the primary group, you must obtain the replica reserve requirement for each primary group volume that you are replicating to the secondary group. When you configure a volume for replication, you specify the replica reserve size as a percentage (minimum 105 percent) of the replica volume reserve, which approximates in-use volume space. As volume changes occur, the replica volume reserve increases; therefore, the replica reserve increases, providing more free space for replicas up to a limit. NOTE: Replica reserve can increase automatically or by administrator action only if free delegated space is available. See Guidelines for Sizing Delegated Space. Ideally, you want to allocate only enough replica reserve to store the desired number of replicas. In general, the higher the replica reserve percentage, the more replicas you can store. However, specifying a high percentage for all volumes might not be the most efficient use of delegated space. The optimal value for replica reserve depends on the volume change rate, which can be difficult to estimate, and the replication frequency. See How Volume Changes Affect Replication Space. Use the following guidelines for sizing replica reserve: • Recommended value Dell recommends that you specify 200 percent for the replica reserve. Specifying 200 percent guarantees that you can store at least two replicas, assuming enough delegated space is available for the replica reserve to reach its maximum size. If the replica reserve cannot reach its maximum size due to lack of delegated space, you are not guaranteed two replicas. If you want to guarantee more than two replicas, specify a higher percentage. • Space-efficient value For volumes that are not frequently modified, you might be able to keep the desired number of replicas by using a replica reserve value that is less than 200 percent. To obtain an appropriate replica reserve value, estimate the average volume change that occurs between replication operations. Then, use the following calculation, where 105 percent is the minimum replica reserve value: 105 percent + [change rate x (number of replicas to keep -1)] For example, if you estimate that at most 20 percent of the volume changes between replication operations, and you want to keep three replicas, specify 145 percent for the replica reserve value. After replication begins, monitor the number of replicas for each volume and the replica reserve usage. If more than the desired number of replicas exist, consider decreasing the replica reserve percentage. If fewer than the desired number of replicas exist, consider increasing the replica reserve percentage. A low free replica reserve can indicate optimal use of replica reserve space, if the desired number of replicas exist. Guidelines for Sizing Delegated Space The secondary group administrator delegates space to the primary group when configuring the group as a replication partner. The secondary group administrator can modify the partner configuration and increase or decrease delegated space as needed. Ideally, you should request from the secondary group administrator only enough delegated space to store the desired number of replicas for each volume. 246 About Backing Up and Protecting Your Data