[jira] [Updated] (SOLR-12509) Improve SplitShardCmd performance and reliability

["Cassandra Targett (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SOLR-12509?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Cassandra Targett updated SOLR-12509:
-------------------------------------
    Component/s: SolrCloud

> 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
>
> {{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 splitting could 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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