Additional factors for sizing the SRL

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 not unlikely that any of these situations could be immediately followed by a period of peak usage. In this case, the Secondary could 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 obtained from the peak usage constraint to the maximum size obtained using the other constraints. Note that this applies even for synchronous RLINKs, which are not normally affected by the peak usage constraint, because after a disconnect, they act as asynchronous RLINKs until caught up.

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

Once an estimate has been computed, one more adjustment must be made to account for the fact that all data 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.

Table: SRL adjustments provides the SLR adjustment percentages required.

Table: SRL adjustments

If average write size is:

Add this percentage to SRL size:

512 bytes

100%

1K

50%

2K

25%

4K

15%

8K

7%

10K

5%

16K

4%

32K or more

2%