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https://issues.apache.org/jira/browse/SOLR-9936?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15808282#comment-15808282
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Mark Miller commented on SOLR-9936:
-----------------------------------
We would want to do some vetting and testing, but perhaps currently okay to
limit the recovery executor.
> Allow configuration for recoveryExecutor thread pool size
> ---------------------------------------------------------
>
> Key: SOLR-9936
> URL: https://issues.apache.org/jira/browse/SOLR-9936
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: replication (java)
> Affects Versions: 6.3
> Reporter: Tim Owen
> Attachments: SOLR-9936.patch
>
>
> There are two executor services in {{UpdateShardHandler}}, the
> {{updateExecutor}} whose size is unbounded for reasons explained in the code
> comments. There is also the {{recoveryExecutor}} which was added later, and
> is the one that executes the {{RecoveryStrategy}} code to actually fetch
> index files and store to disk, eventually calling an {{fsync}} thread to
> ensure the data is written.
> We found that with a fast network such as 10GbE it's very easy to overload
> the local disk storage when doing a restart of Solr instances after some
> downtime, if they have many cores to load. Typically we have each physical
> server containing 6 SSDs and 6 Solr instances, so each Solr has its home dir
> on a dedicated SSD. With 100+ cores (shard replicas) on each instance,
> startup can really hammer the SSD as it's writing in parallel from as many
> cores as Solr is recovering. This made recovery time bad enough that replicas
> were down for a long time, and even shards marked as down if none of its
> replicas have recovered (usually when many machines have been restarted). The
> very slow IO times (10s of seconds or worse) also made the JVM pause, so that
> disconnects from ZK, which didn't help recovery either.
> This patch allowed us to throttle how much parallelism there would be writing
> to a disk - in practice we're using a pool size of 4 threads, to prevent the
> SSD getting overloaded, and that worked well enough to make recovery of all
> cores in reasonable time.
> Due to the comment on the other thread pool size, I'd like some comments on
> whether it's OK to do this for the {{recoveryExecutor}} though?
> It's configured in solr.xml with e.g.
> {noformat}
> <updateshardhandler>
> <int name="maxRecoveryThreads">${solr.recovery.threads:4}</int>
> </updateshardhandler>
> {noformat}
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