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https://issues.apache.org/jira/browse/SOLR-12509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16550662#comment-16550662
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Shalin Shekhar Mangar commented on SOLR-12509:
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Awesome speedups!

A few minor issues:
# SolrIndexSplitter.findDocsToDelete uses the wrong key to lookup inside the 
synchronized block -- {{docsToDelete.get(readerContext.ord);}}
# There is a new {{DefaultSolrCoreState.getIndexWriterLock}} method which isn't 
used anywhere?
# Typo {{changepostd}} in {{ReplicaMutator}}
# We should rename {{index.split}} to follow the {{index.<timestamp>}} 
convention otherwise dangling "index.split" directories won't be cleaned up by 
{{DirectoryFactory.cleanupOldIndexDirectories}}

> 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
>
>
> {{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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