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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7139) Default concurrent_compactors is probably too high
Date Fri, 02 May 2014 15:26:14 GMT


Benedict commented on CASSANDRA-7139:

How about: 1 per disk, with a cap of 8, say? Boxes with 12+ (even 24+) disks aren't totally
uncommon and you could see the same problem there as well. 

This should all be less of a problem with CASSANDRA-6696 as we'll be able to actually schedule
on a per-disk basis and have no risk of referring to files on other disks, so we just want
a sensible number to avoid breaking anyone who hasn't tuned their nodes between now and then.

> Default concurrent_compactors is probably too high
> --------------------------------------------------
>                 Key: CASSANDRA-7139
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Priority: Minor
>             Fix For: 2.0.8, 2.1 rc1
> The default number of concurrent compactors is probably too high for modern hardware
with spinning disks for storage: A modern blade can easily have 24+ Cores, which would result
in a default of 24 concurrent compactions. This not only increases random IO, it also keeps
around a lot of obsoleted files for an unnecessarily long time, as each compaction keeps references
to any possibly overlapping files that it isn't itself compacting - but these can have been
obsoleted part way through by compactions that finished earlier. If you factor in the default
compaction throughput rate of 16Mb/s, anything but a single default concurrent_compactor makes
very little sense, as a single thread should always be able to handle 16Mb/s, will cause less
interference with other processes, and permits obsoleted files to be immediately removed.
> See [] for a graph demonstrating the result of making this change
on a box with 24-cores and 8Tb of storage (first spike is default settings)

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