A very important consideration is the capacity sizing of your RecoverPoint journals which is done on a per Consistency Group basis. This will dicatate the level of failback allowed (Protection Window) on a per CG configuration. The JV must have the correct performance characteristics in order to handle the total write performance of the LUN(s) being protected (next blog). They must also have the capacity to store all the writes of the protected LUN(s). The two most important questions to ask are:

• What change rate does the source LUN generate?
• What retention window is required?

To calculate the journal capacity, you must measure the rate of change on the production LUNs within a CG. This can be done by analyzing the data per second values using the Array analysis tools (for example on the EMC VNX array, data per second values are found using the Unisphere Analyzer).This figure may change over the course of say a 24 hour period so it is good practice to average out the rate of change requirement. Also note that EMC as well as partners have access to EMC Business Continuity Solution Designer (BCSD) to help in sizing journals.

The Journal Volume Sizing formula is:
Journal size= (data per second Mbps) * (required rollback time in seconds)/(1- target side log size) x 1.05

Twenty percent of the journal must be reserved for the target side log and five percent for internal system needs.
For example, to support a 24-hour rollback requirement (86,400 seconds), with 10 megabits per second (Mb/s) of new data writes to the replication volumes in a consistency group, the calculation is:

(10 * 86,400)/(1- 0.2) x 1.05 = 1,134,000Mb = ~140GB

The example here is to give you an idea of the planning behind the capacity requirements. While the above calculation is used for sizing capacity of the Journal my next blog will detail the performance characteristics required by both the Journal and Replica volumes.

Note: When the change rate values are not available then a general rule of thumb is to size the journal at 20% of the data being replicated.