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From "Stu Hood (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-2191) Multithread across compaction buckets
Date Thu, 31 Mar 2011 05:41:05 GMT


Stu Hood commented on CASSANDRA-2191:

Regarding 6.: there is an issue related to behaviour here: thank you for pointing it out.
As implemented in this patch, triggering a major compaction will start it immediately with
whatever sstables aren't already active in compaction: this means that triggering a major
compaction while another compaction is running will not result in a major compaction. In the
past, this wouldn't have been kosher, because major compactions were required to clean up

But now that we have the bloomfilter checking optimization for compaction, the new behaviour
is probably sufficient: when there ''aren't'' any other compactions active, the "major" flag
is an optimization that makes the bloomfilter checks unnecessary; when there ''are'' other
compactions active, the bloomfilter check works as usual.


> Multithread across compaction buckets
> -------------------------------------
>                 Key: CASSANDRA-2191
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Priority: Critical
>              Labels: compaction
>             Fix For: 0.8
>         Attachments: 0001-Add-a-compacting-set-to-sstabletracker.txt, 0002-Use-the-compacting-set-of-sstables-to-schedule-multith.txt,
> This ticket overlaps with CASSANDRA-1876 to a degree, but the approaches and reasoning
are different enough to open a separate issue.
> The problem with compactions currently is that they compact the set of sstables that
existed the moment the compaction started. This means that for longer running compactions
(even when running as fast as possible on the hardware), a very large number of new sstables
might be created in the meantime. We have observed this proliferation of sstables killing
performance during major/high-bucketed compactions.
> One approach would be to pause compactions in upper buckets (containing larger files)
when compactions in lower buckets become possible. While this would likely solve the problem
with read performance, it does not actually help us perform compaction any faster, which is
a reasonable requirement for other situations.
> Instead, we need to be able to perform any compactions that are currently required in
parallel, independent of what bucket they might be in.

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