Volume Replicator 7.4.1 Administrator's Guide - Windows

Additional factors

Once estimates of required SRL size have been obtained under each of the constraints described above, several additional factors must be considered.

For the synchronization period, downtime and Secondary backup constraints, it is likely that a period of peak usage may follow any of these situations. In this case, the Secondary can continue to fall further behind rather than catching up during the peak usage period. As a result, it might be necessary to add the size that is obtained from the peak usage constraint to the maximum size that is obtained using the other constraints. Note that this applies even for synchronous RLINKs, which are not normally affected by the peak usage constraint as after a disconnect they act as asynchronous RLINKs until caught up.

It is also possible that other situations can occur requiring additions to constraints. For example, a long network failure can follow a synchronization period or a Secondary node failure can follow a network failure. Whether and to what degree to plan for unlikely occurrences requires weighing the cost of additional storage against the cost of additional downtime that is caused by SRL overflow.

Once an estimate has been computed, one more adjustment must be made to account for the fact that all data that is written to the SRL also includes some header information. This adjustment must take into account the typical size of write requests. Each request uses at least one additional disk block for header information.

For AIX, Linux, and Solaris, the adjustments are as follows:

For HP-UX, the adjustments are as follows: