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https://issues.apache.org/jira/browse/CASSANDRA-5371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13622118#comment-13622118
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Jonathan Ellis commented on CASSANDRA-5371:
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bq. Basically I wonder if there won't be a number of scenario where because you
get a bit behind on L0 once, then the I/O you "waste" doing STSC in L0 will
help you get even more and more behind on your leveling
That's exact;y the case, which is why we only apply STCS to L0 when it's fairly
badly behind, i.e., we can conclude two things:
# if the current workload continues, it's not going to magically catch up any
time soon
# reads are starting to get into trouble
Note that #2 will cause a vicious cycle, slowing down compaction in turn.
So while I can hypothesize workloads that burst just long enough to cause STCS
to kick in before stopping, thus "wasting" iops, I think for the vast majority
this is a good "safety valve," and specifically not worth adding a config
option to disable.
However, I do think it's worth creating a ticket to allow STCS config options
to be applied to the size-tiering done by LCS, and specifically allow
configuring MAX_COMPACTION_L0 via the max sstables threshold, which I think may
adequately address your concern.
> Perform size-tiered compactions in L0
> -------------------------------------
>
> Key: CASSANDRA-5371
> URL: https://issues.apache.org/jira/browse/CASSANDRA-5371
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Jonathan Ellis
> Assignee: Jonathan Ellis
> Fix For: 2.0
>
> Attachments: HybridCompactionStrategy.java
>
>
> If LCS gets behind, read performance deteriorates as we have to check bloom
> filters on man sstables in L0. For wide rows, this can mean having to seek
> for each one since the BF doesn't help us reject much.
> Performing size-tiered compaction in L0 will mitigate this until we can catch
> up on merging it into higher levels.
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