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https://issues.apache.org/jira/browse/CASSANDRA-11119?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15131501#comment-15131501
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Jonathan Ellis commented on CASSANDRA-11119:
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We can actually get pretty accurate with STCS thanks to HyperLogLog.
For LCS we're probably still looking at some pretty rough estimates.
> Add bytes-to-compact estimate
> -----------------------------
>
> Key: CASSANDRA-11119
> URL: https://issues.apache.org/jira/browse/CASSANDRA-11119
> Project: Cassandra
> Issue Type: Bug
> Components: Compaction
> Reporter: Jonathan Ellis
> Fix For: 3.x
>
>
> Pending tasks is essentially useless at letting operators know how far behind
> compaction is. For STCS, because compaction tasks can vary in size by
> several orders of magnitude, so "4 more tasks" is a very wide range. For
> LCS, I think the problem is that we only look at first-order work (how much
> larger is level X than it "should" be?) and not cascading effects,
> particularly in L0 (if we push this into L1, how much does that push into L2
> etc?).
> I propose adding an estimate of bytes-to-compact (and deprecating estimated
> tasks).
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