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https://issues.apache.org/jira/browse/SOLR-12509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16565690#comment-16565690
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ASF subversion and git services commented on SOLR-12509:
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Commit 7faa803a7c9699f38b8a6b3ddd3a88c4729c5e5f in lucene-solr's branch 
refs/heads/branch_7x from Andrzej Bialecki
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=7faa803 ]

SOLR-12509: Improve SplitShardCmd performance and reliability.


> Improve SplitShardCmd performance and reliability
> -------------------------------------------------
>
>                 Key: SOLR-12509
>                 URL: https://issues.apache.org/jira/browse/SOLR-12509
>             Project: Solr
>          Issue Type: Improvement
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrCloud
>            Reporter: Andrzej Bialecki 
>            Assignee: Andrzej Bialecki 
>            Priority: Major
>         Attachments: SOLR-12509.patch, SOLR-12509.patch
>
>
> {{SplitShardCmd}} is currently quite complex.
> Shard splitting occurs on active shards, which are still being updated, so 
> the splitting has to involve several carefully orchestrated steps, making 
> sure that new sub-shard placeholders are properly created and visible, and 
> then also applying buffered updates to the split leaders and performing 
> recovery on sub-shard replicas.
> This process could be simplified in cases where collections are not actively 
> being updated or can tolerate a little downtime - we could put the shard 
> "offline", ie. disable writing while the splitting is in progress (in order 
> to avoid users' confusion we should disable writing to the whole collection).
> The actual index splittingĀ couldĀ perhaps be improved to use 
> {{HardLinkCopyDirectoryWrapper}} for creating a copy of the index by 
> hard-linking existing index segments, and then applying deletes to the 
> documents that don't belong in a sub-shard. However, the resulting index 
> slices that replicas would have to pull would be the same size as the whole 
> shard.



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