Question
Julian Matthews · Nov 5, 2020

Impact of Journal File Size on MirrorDatabaseLatencyTime

Hey everyone.

I have noticed that my backup mirror is warning that the MirrorDatabaseLatencyTime is having a bad time (time in ms is 3000, and warnvalue is 3000). While I look into what may be causing this latency between the two servers, I was considering if reducing the size of the journal files would improve this value in any way.

My assumption is that reducing the file size would mean that the frequency of the journal files being created would be increased, but the reduced size would mean that the transfer and application of each file would be reduced.

Has anyone had experience in this area?

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Replies

Hi Julian, I don't believe changing the journal sizes would have an impact in this case because the message indicates the slowness is most likely related to applying the data to the mirror databases, not transferring it from the primary.  The reason I say this is because there are two separate System Monitor alerts you could have:

  MirrorJournalLatencyTime:  time between when the last journal file was received from the primary and when it was written to the journal disk.

  MirrorDatabaseLatencyTime:  time between when the last journal file was received from the primary and when it was applied to the mirror database(s).

So in your case the backup's mirror mechanism that reads the copied journal data and then applies it to the mirror databases does not seem to be able to keep up with the rate of updates from the primary.  If it were due to slowness copying journals, you should be seeing the MirrorJournalLatencyTime alert instead.  You can read these definitions in the System Monitor documentation:

  https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls?KEY=GCM_healthmon

If this continues to be an issue I recommend contacting Support (617-621-0700 or Support@InterSystems.com) and we can help to investigate this behavior.

Hey Andrew. Good catch, I completely mixed the two up.

Looks like WRC will be getting my call this morning smiley