Log I/O count differences – timing is everything

While reviewing a customer’s MQ for z/OS SMF statistics data, I noticed that the LOG_CI count did not always match the individual log copy I/O counts. When I asked level 3 about this discrepancy, it turns out it is a difference in timing. The LOG_IO (QJSTCIWR) is updated before the write is started, the individual I/O counts are updated after the I/O has completed. So when the system is very active with persistent messages the counts may be off a little – typically just 1-2 depending on whether single or dual logging is used.

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