[
https://issues.apache.org/jira/browse/CASSANDRA-7139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14266473#comment-14266473
]
Catalin Alexandru Zamfir commented on CASSANDRA-7139:
-----------------------------------------------------
Our set-up was RAID5 and the min (numberOfDisk, numberOfCores) would just be 2,
when we have 40+ cores. The commented "concurrent_compactors" would be "2"
meaning that a lot of SSTables are accumulating in high-cardinality tables
(where the partition key is an UUID-type) because the compaction is limited to
"2". Looking at "dstat" even if we've set compaction_throughput_in_mb_per_sec
to 192 (spinning disk) the dstat -lrv1 disk write maxes out at 10MB/s.
IMHO, the concurrent_compactors should be
number_of_cores/compaction_throughput_in_mb_per_sec * 100 which in our case (40
cores) gives around 20/21 compactors. And on 8 cores (8/192 * 100 gives 4
concurrent compactors).
> Default concurrent_compactors is probably too high
> --------------------------------------------------
>
> Key: CASSANDRA-7139
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7139
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Benedict
> Assignee: Jonathan Ellis
> Priority: Minor
> Fix For: 2.1 rc1
>
> Attachments: 7139.txt
>
>
> 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 [http://imgur.com/HDqhxFp] 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)
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
