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From "Albert P Tobey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9274) Changing memtable_flush_writes per recommendations in cassandra.yaml causes memtable_cleanup_threshold to be too small
Date Thu, 11 Jun 2015 06:03:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9274?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14581497#comment-14581497
] 

Albert P Tobey commented on CASSANDRA-9274:
-------------------------------------------

I've been messing with these values lately and have observed some poor behavior around them.
The comment in cassandra.yaml is misleading.

I agree we shouldn't set the threshold by default, but I would like to see a comment added
to memtable_flush_writers indicating that if you add a large number of flush writers, the
default memtable_cleanup_threshold is going to end up small, which leads to small flushes
and more frequent compaction.

It makes some sense to set the memtable_cleanup_threshold based on the expected number of
tables rather than cores or disks. I might be wrong, but that seems more relevant than the
hardware or even the number of flush writer threads.

> 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.



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