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https://issues.apache.org/jira/browse/SOLR-10277?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15926140#comment-15926140
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Joshua Humphries commented on SOLR-10277:
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Oops. I was wrong about needing the LinkedHashSet in ClusterState.
Looks like I solved the deterministic ordering issue twice and left both of the
fixes in the patch. The other fix is where the test sorts the list of writes,
and that alone suffices. So we can undo those changes in ClusterState.java.
> On 'downnode', lots of wasteful mutations are done to ZK
> --------------------------------------------------------
>
> Key: SOLR-10277
> URL: https://issues.apache.org/jira/browse/SOLR-10277
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: 5.5.3, 5.5.4, 6.0.1, 6.2.1, 6.3, 6.4.2
> Reporter: Joshua Humphries
> Labels: leader, zookeeper
> Attachments: SOLR-10277-5.5.3.patch
>
>
> When a node restarts, it submits a single 'downnode' message to the
> overseer's state update queue.
> When the overseer processes the message, it does way more writes to ZK than
> necessary. In our cluster of 48 hosts, the majority of collections have only
> 1 shard and 1 replica. So a single node restarting should only result in
> ~1/40th of the collections being updated with new replica states (to indicate
> the node that is no longer active).
> However, the current logic in NodeMutator#downNode always updates *every*
> collection. So we end up having to do rolling restarts very slowly to avoid
> having a severe outage due to the overseer having to do way too much work for
> each host that is restarted. And subsequent shards becoming leader can't get
> processed until the `downnode` message is fully processed. So a fast rolling
> restart can result in the overseer queue growing incredibly large and nearly
> all shards winding up in a leader-less state until that backlog is processed.
> The fix is a trivial logic change to only add a ZkWriteCommand for
> collections that actually have an impacted replica.
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