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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-7031) Increase default commit log total space + segment size
Date Mon, 14 Apr 2014 10:17:15 GMT
Benedict created CASSANDRA-7031:
-----------------------------------

             Summary: Increase default commit log total space + segment size
                 Key: CASSANDRA-7031
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7031
             Project: Cassandra
          Issue Type: Improvement
          Components: Core
            Reporter: Benedict
            Assignee: Benedict
            Priority: Trivial
             Fix For: 2.1 beta2


I would like to increase the default commit log total space and segment size options for 64-bit
JVMs:

The current default of 1Gb and 32Mb is quite constrained and can have some (very minor) negative
performance implications, for no major benefit: 

# 32Mb files are actually quite small, and if during the 10s interval we have completely filled
multiple of them (quite easy) it would be more efficient to write fewer larger files, as we
can issue fewer fsyncs and permit the OS to schedule the writes more efficiently. On my box
this has a small but noticeable impact. Although I would expect on decent server hardware
this would be smaller still, since we immediately drop the pages from cache on writing there
isn't a great deal of advantage to keeping the files so small. The only advantage I can see
is that during a drop KS/CF or other event that forces log rollover we're wasting less space
until log recycling. 128-256Mb are modest increases that seem more appropriate to me.

# 1Gb is too small for the default total log space. We can find that we force memtable flushes
as a result of log utilisation instead of memtable occupancy quite often (esp. as a result
of increased effective memtable space from recent improvements), especially on machines with
more addressable memory. I suggest 8Gb as a minimum. The only disadvantage of having more
log data is that replay on restart may be slightly slower, but since most of the events will
be ignored it should be relatively benign, and I would rather take the penalty on startup
instead of during running, no matter how small the running penalty.





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