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From "Kurt Greaves (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-12730) Thousands of empty SSTables created during repair - TMOF death
Date Mon, 07 Nov 2016 22:54:58 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-12730?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15645739#comment-15645739
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Kurt Greaves commented on CASSANDRA-12730:
------------------------------------------

For the record, it seems to me like this is a potentially harmless optimisation, but doesn't
really fix the core issue.

 I've seen similar issues on a lot of clusters on a lot of different versions (pretty much
every version since 2.1.11). This issue occurs quite frequently and not necessarily on clusters
under heap pressure. Usually when we see this issue the logs report masses of streaming sessions
and flushes. The proposed solution seems to be simple and harmless enough even though it may
not fix the real issue and I think would help in cases where there are a small amount of constant
writes to a table such that the memtable is never/rarely clean when the flush is triggered,
resulting in lots of small SSTables. 

I think the real issue is a problem elsewhere that triggers lots of stream sessions during
the repair even though the data is consistent. We've seen this sort of thing in cases where
we're fairly sure there have been no down nodes and no dropped mutations, and even running
repairs continuously results in many streams.



> Thousands of empty SSTables created during repair - TMOF death
> --------------------------------------------------------------
>
>                 Key: CASSANDRA-12730
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12730
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Local Write-Read Paths
>            Reporter: Benjamin Roth
>            Priority: Critical
>
> Last night I ran a repair on a keyspace with 7 tables and 4 MVs each containing a few
hundret million records. After a few hours a node died because of "too many open files".
> Normally one would just raise the limit, but: We already set this to 100k. The problem
was that the repair created roughly over 100k SSTables for a certain MV. The strange thing
is that these SSTables had almost no data (like 53bytes, 90bytes, ...). Some of them (<5%)
had a few 100 KB, very few (<1% had normal sizes like >= few MB). I could understand,
that SSTables queue up as they are flushed and not compacted in time but then they should
have at least a few MB (depending on config and avail mem), right?
> Of course then the node runs out of FDs and I guess it is not a good idea to raise the
limit even higher as I expect that this would just create even more empty SSTables before
dying at last.
> Only 1 CF (MV) was affected. All other CFs (also MVs) behave sanely. Empty SSTables have
been created equally over time. 100-150 every minute. Among the empty SSTables there are also
Tables that look normal like having few MBs.
> I didn't see any errors or exceptions in the logs until TMOF occured. Just tons of streams
due to the repair (which I actually run over cs-reaper as subrange, full repairs).
> After having restarted that node (and no more repair running), the number of SSTables
went down again as they are compacted away slowly.
> According to [~zznate] this issue may relate to CASSANDRA-10342 + CASSANDRA-8641



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