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Posted to issues@solr.apache.org by "David Smiley (Jira)" <ji...@apache.org> on 2023/02/27 17:31:00 UTC
[jira] [Commented] (SOLR-16438) Shard split should be able to set preferred leaders on other replicas
[ https://issues.apache.org/jira/browse/SOLR-16438?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17694099#comment-17694099 ]
David Smiley commented on SOLR-16438:
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[~broustant] : Recently we saw that the precondition of {{!isPreferredLeaderCurrentLeader(collection, shardName, h))}} (then do a rebalance) was not working; led to hangs. Apparently there was no test exercising this code path? If it's not fixed, then this would need to be reverted (same JIRA; unreleased code). It'd kind of sad because we want to split at the correct leader.
> 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
> 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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