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From jahlborn <jahlb...@gmail.com>
Subject Re: Can you change the kahadb journal file size between broker starts
Date Fri, 20 Jan 2017 14:58:44 GMT
So, i was poking around to see the actual implications of changing the max
journal file length, and i must say, the code doesn't seem to back up the
claim that it is a safe thing to do.

https://fisheye.apache.org/browse/activemq-6/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/disk/journal/Journal.java?hb=true#to310

This is the startup code.  it attempts to adjust the total length by shaving
off an unused portion of the final data file.  if you have made the
maxFileLength smaller since the last run, and you have actual data in the
last journal file which is after the new maxFileLength, then this
computation will incorrectly return a negative value.  my suspicion is that
the length of the last data file should be used here instead of
maxFileLength.

https://fisheye.apache.org/browse/activemq-6/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/disk/journal/Journal.java?hb=true#to867

likewise, this code attempts to handle adjustments to maxFileLength since
the journal file was created, but i think it again fails.  if the
maxFileLength has been increased since the data file was created, this would
seem to be setting an offset which is past the length of the current data
file.  again, it seems like the length of the data file should be used
directly.

in general, in order for this to work correctly, the maxFileLength should
only be used for code which is creating new files.  all the other code
should be solely relying on the size of the existing file, right?



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