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https://issues.apache.org/jira/browse/SOLR-15055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17262452#comment-17262452
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Andrzej Bialecki commented on SOLR-15055:
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[~mdrob] in 8x `maxShardsPerNode` was totally broken and didn't do what you
would expect it to. It didn't actually affect the placement at all - it was
only a crude initial check to see if the total number of replicas was larger
than the number of nodes x maxShardsPerNode, regardless of where the placements
would be. That's why almost all tests would set it to -1 or a large number,
simply to bypass this check.
> Re-implement 'withCollection' and 'maxShardsPerNode'
> ----------------------------------------------------
>
> Key: SOLR-15055
> URL: https://issues.apache.org/jira/browse/SOLR-15055
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
> Time Spent: 4h 20m
> Remaining Estimate: 0h
>
> Solr 8x replica placement provided two settings that are very useful in
> certain scenarios:
> * {{withCollection}} constraint specified that replicas should be placed on
> the same nodes where replicas of another collection are located. In the 8x
> implementation this was limited in practice to co-locating single-shard
> secondary collections used for joins or other lookups from the main
> collection (which could be multi-sharded).
> * {{maxShardsPerNode}} - this constraint specified the maximum number of
> replicas per shard that can be placed on the same node. In most scenarios
> this was set to 1 in order to ensure fault-tolerance (ie. at most 1 replica
> of any given shard would be placed on any given node). Changing this
> constraint to values > 1 would reduce fault-tolerance but may be desired in
> test setups or as a temporary relief measure.
>
> Both these constraints are collection-specific so they should be configured
> e.g. as collection properties.
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