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From "Branimir Lambov (Jira)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-16782) Improve the way we pick sstables for STCS-in-L0 and in TWCS 'current' window
Date Tue, 03 Aug 2021 14:59:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-16782?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17392350#comment-17392350
] 

Branimir Lambov commented on CASSANDRA-16782:
---------------------------------------------

I have some general questions about the improvement. It looks like it is a combination of
two things:
 * A drastic increase of STCS's limit for max sstables in a compaction, and
 * A preference of the STCS level to compact based on the number of sstables,

which only apply to uses of STCS in LCS and TWCS.

I can see a lot of value in the latter, but why not apply it always, including plain STCS?
STCS's selection of bucket to compact does result in accumulation of sstables on the smallest-sstables
bucket and at DataStax we were recently discussing selecting the most populous bucket as a
possible solution to this problem, which might be good enough even without raising the max
limit. I personally could not find any downsides to it, and I wonder why you would prefer
to restrict its usage?

The former we can achieve by configuration, can't we? (Perhaps also adding a max size limit.)
Speaking of which, is there a reason to place the new settings in {{cassandra.yaml}} instead
of compaction parameters?

> Improve the way we pick sstables for STCS-in-L0 and in TWCS 'current' window
> ----------------------------------------------------------------------------
>
>                 Key: CASSANDRA-16782
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-16782
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Local/Compaction/LCS, Local/Compaction/TWCS
>            Reporter: Marcus Eriksson
>            Assignee: Marcus Eriksson
>            Priority: Normal
>             Fix For: 4.x
>
>
> The goal when being behind in L0 should always be to get the number of sstables down
to a reasonable level as soon as possible. Currently it is common that we run compactions
on the large sstables but leave thousands of tiny sstables behind.



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