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https://issues.apache.org/jira/browse/SOLR-16438?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17677926#comment-17677926
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Houston Putman commented on SOLR-16438:
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{{SplitShardTest.testConcurrentSplitThreeHostsRepFactorTwo()}} (which uses the
preferred leaders option) is still failing at a rate of 12%
> Shard split should be able to set preferred leaders on other replicas
> ---------------------------------------------------------------------
>
> Key: SOLR-16438
> URL: https://issues.apache.org/jira/browse/SOLR-16438
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Bruno Roustant
> Assignee: Bruno Roustant
> Priority: Major
> Fix For: 9.2
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> Currently, shard split always create a first replica for each sub-shard on
> the current host. Then it creates other replicas and their corresponding
> sub-shards are in RECOVERY state. The effect is that the first replica (on
> the current host) is always the leader, meaning that if the sub-shards are
> split themselves, their sub-sub-shards leaders are also on the same host.
> This can lead to very unbalanced situation where the same host is the leader
> for a whole set of shards.
> A solution to distribute evenly the leaders is to flag some other replicas
> with the preferredLeader property during the split. Then a rebalance-leaders
> command can elect the appropriate leaders. If we do that for each split, then
> all the sub-shards have their leaders correctly balanced.
> To go further, we can improve CollectionsHandler#CollectionOperation to
> support combined operations. That way a CollectionOperation#SPLITSHARD_OP can
> trigger a split op, then a wait for split completion op, and then a rebalance
> leaders op.
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