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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SOLR-12730) Implement staggered SPLITSHARD requests in IndexSizeTrigger
Date Tue, 04 Sep 2018 07:48:02 GMT
Andrzej Bialecki  created SOLR-12730:
----------------------------------------

             Summary: Implement staggered SPLITSHARD requests in IndexSizeTrigger
                 Key: SOLR-12730
                 URL: https://issues.apache.org/jira/browse/SOLR-12730
             Project: Solr
          Issue Type: Improvement
      Security Level: Public (Default Security Level. Issues are Public)
          Components: AutoScaling
            Reporter: Andrzej Bialecki 
            Assignee: Andrzej Bialecki 
             Fix For: master (8.0)


Simulated large scale tests uncovered an interesting scenario that occurs also in real clusters
where {{IndexSizeTrigger}} is used for controlling the maximum shard size.

As index size grows and the number of shards grows, if document assignment is more or less
even then at equal intervals (on a {{log2}} scale) there will be an avalanche of SPLITSHARD
operations, because all shards will reach the critical size at approximately the same time.

A hundred or more split shard operations running in parallel may severely affect the cluster
performance.

One possible approach to reduce the likelihood of this situation is to split shards not exactly
in half but rather fudge the proportions around 60/40% in a random sequence, so that the resulting
sub-sub-sub…shards would reach the thresholds at different times. This would require modifications
to the SPLITSHARD command to allow this randomization.

Another approach would be to simply limit the maximum number of parallel split shard operations.
However, this would slow down the process of reaching the balance (increase lag) and possibly
violate other operational constraints due to some shards waiting too long for the split and
significantly exceeding their max size.



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