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From "Donald Smith (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-9274) Changing memtable_flush_writes per recommendations in cassandra.yaml causes memtable_cleanup_threshold to be too small
Date Thu, 30 Apr 2015 19:24:07 GMT
Donald Smith created CASSANDRA-9274:
---------------------------------------

             Summary: Changing memtable_flush_writes per recommendations in cassandra.yaml
causes  memtable_cleanup_threshold to be too small
                 Key: CASSANDRA-9274
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9274
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Donald Smith
            Priority: Minor


It says in cassandra.yaml:
{noformat}
# If your data directories are backed by SSD, you should increase this
# to the number of cores.
#memtable_flush_writers: 8
{noformat}
so we raised it to 24.

Much later we noticed a warning in the logs:
{noformat}
WARN  [main] 2015-04-22 15:32:58,619 DatabaseDescriptor.java:539 - memtable_cleanup_threshold
is set very low, which may cause performance degradation
{noformat}
Looking at cassandra.yaml again I see:
{noformat}
# memtable_cleanup_threshold defaults to 1 / (memtable_flush_writers + 1)
# memtable_cleanup_threshold: 0.11
#memtable_cleanup_threshold: 0.11
{noformat}
So, I uncommented that last line (figuring that 0.11 is a reasonable value).

Cassandra.yaml should give better guidance or the code should *prevent* the value from going
outside a reasonable range.
{noformat}



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