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https://issues.apache.org/jira/browse/CASSANDRA-1972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12981625#action_12981625
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Stu Hood commented on CASSANDRA-1972:
-------------------------------------
Based on IO/seek bound tests on two machines with 4 and 12 drives in RAID-0 and
RAID-10, we saw the best performance with around 20 threads per drive. Not
everyone will be completely IO bound though (hopefully _something_ is in
cache), so perhaps a better recommended value is 16 threads per drive. I'll
write a new blurb for the config file.
> Default concurrency values are improperly proportioned
> ------------------------------------------------------
>
> Key: CASSANDRA-1972
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1972
> Project: Cassandra
> Issue Type: Bug
> Reporter: Stu Hood
> Assignee: Stu Hood
> Priority: Minor
> Fix For: 0.7.1
>
>
> The "default"/"suggested" {{concurrent_reads}} value is much too low. It
> assumes that CPU will be the bottleneck, rather than IO, and for most
> deployments, this will not be the case. Additionally it is better to be
> queued for IO in the kernel or on your device than in user space, because the
> former work to optimize queue order.
> Additionally, reads are much cheaper than writes in terms of CPU time (since
> writes can experience contention due to retries), so while
> {{concurrent_writes}} should probably factor in the number of cores on the
> machine, {{concurrent_reads}} should probably be calculated purely by number
> of spindles.
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