Frequent page set backup is essential if a reasonably short recovery time is required. This applies even when a page set is very small or there is a small amount of activity on queues in that page set.
If you use persistent messages in a page set, the backup frequency should be in hours rather than days. This is also the case for page set zero.
To calculate an approximate backup frequency, start by determining the target total recovery time. This consists of the following:
(For example, you can restore approximately 60 cylinders of 3390 data a minute from and to RAMAC Virtual Array 2 Turbo 82 (RVA2-T82) DASD using Access Method Services REPRO.)
If you use SnapShot backup/restore, the time taken to perform this task is a few seconds. For information about SnapShot, see the DFSMSdss Storage Administration Guide.
This depends most significantly on the amount of log data that must be read from active and archive logs since that page set was last backed up. All such log data must be read, in addition to that directly associated with the damaged page set.
When deciding on how long to allow for the recovery of the page set, the factors that you need to consider are:
The rate at which data is written to the log depends on how messages arrive in your system, in addition to the message rate. Messages received or sent over a channel result in more data logging than messages generated and retrieved locally.
When reading the logs, the achievable data rate depends on the devices used and the total load on your particular DASD subsystem. (For example, data rates of approximately 2.7 MB a second have been observed using active and archive logs on RVA2-T82 DASD.)
With most tape units, it is possible to achieve higher data rates for archived logs with a large block size. However, if an archive log is required for recovery, all the data on the active logs must be read also.
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