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https://issues.apache.org/jira/browse/SOLR-13396?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17682640#comment-17682640
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David Smiley commented on SOLR-13396:
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Just reviewed the description & conversation here. It seems the scenario to
guard against is the wrong ZK being referenced (for whatever reason, to include
ZK getting wiped clean). In such a scenario *every* core on disk will not be
linked to a replica in ZK. If this happens, I think we can, by default, take
no deletion action. Perhaps even shutdown Solr. Configuration options to
toggle this are okay but it'd be nice to improve the default behavior because
people won't find this configuration option and use it unless they've been
burned :)
> 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
> Components: SolrCloud
> Affects Versions: 7.3.1, 8.0
> Reporter: Shawn Heisey
> Priority: Major
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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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