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How often should a page set be backed up?
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:
- The time taken to react to the problem.
- The time taken to restore the page set backup copy.
(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.
- The time the queue manager requires to restart, including the additional time needed to recover the page set.
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.
Note:When using fuzzy backup (where a snapshot is taken of the logs and page sets while a unit of work is active), it might be necessary to read up to three additional checkpoints, and this might result in the need to read one or more additional logs.
When deciding on how long to allow for the recovery of the page set, the factors that we need to consider are:
- The rate at which data is written to the active logs during normal processing:
- Approximately 1.3 KB of extra data is required on the log for a persistent message.
- Approximately 2.5 KB of data is required on the log for each batch of messages sent on a channel.
- Approximately 1.4 KB of data is required on the log for each batch of messages received on a channel.
- Nonpersistent messages require no log data. NPMSPEED(FAST) channels require no log data for batches consisting entirely of nonpersistent messages.
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.
- The rate at which data can be read from the archive and active logs.
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.