Tim Owen created SOLR-9936:
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Summary: 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
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).
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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