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https://issues.apache.org/jira/browse/SOLR-13396?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16822603#comment-16822603
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Gus Heck commented on SOLR-13396:
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I think it's probably sufficient to add a long delay before deletion. On 
startup and once every hour compare the state and the collections on disk, 
anything that's not had representation in the zookeeper state for 24h can be 
deleted. (Time period should be configurable) The thing that's the problem here 
is that the moment you start it with bad settings your days long ingest of 50 
billion documents goes bye bye. That's very different than starting it and not 
noticing that you can't see any collections for 24 hours... I just had to 
explain zkchroot to a customer with a data set of that magnitude... so this 
issue is kinda scary to me :)

> SolrCloud will delete the core data for any core that is not referenced in 
> the clusterstate
> -------------------------------------------------------------------------------------------
>
>                 Key: SOLR-13396
>                 URL: https://issues.apache.org/jira/browse/SOLR-13396
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrCloud
>    Affects Versions: 7.3.1, 8.0
>            Reporter: Shawn Heisey
>            Priority: Major
>
> SOLR-12066 is an improvement designed to delete core data for replicas that 
> were deleted while the node was down -- better cleanup.
> In practice, that change causes SolrCloud to delete all core data for cores 
> that are not referenced in the ZK clusterstate.  If all the ZK data gets 
> deleted or the Solr instance is pointed at a ZK ensemble with no data, it 
> will proceed to delete all of the cores in the solr home, with no possibility 
> of recovery.
> I do not think that Solr should ever delete core data unless an explicit 
> DELETE action has been made and the node is operational at the time of the 
> request.  If a core exists during startup that cannot be found in the ZK 
> clusterstate, it should be ignored (not started) and a helpful message should 
> be logged.  I think that message should probably be at WARN so that it shows 
> up in the admin UI logging tab with default settings.



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