[jira] [Commented] (SOLR-11815) TLOG leaders going down and rejoining as a replica do fullCopy when not needed

["Erick Erickson (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-11815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16986397#comment-16986397 ] 

Erick Erickson commented on SOLR-11815:
---------------------------------------

[~ichattopadhyaya] Did SOLR-11920 fix this and, if so, can we close this?

> TLOG leaders going down and rejoining as a replica do fullCopy when not needed
> ------------------------------------------------------------------------------
>
>                 Key: SOLR-11815
>                 URL: https://issues.apache.org/jira/browse/SOLR-11815
>             Project: Solr
>          Issue Type: Bug
>          Components: replication (java)
>    Affects Versions: 7.2
>         Environment: Oracle JDK 1.8
> Ubuntu 16.04
>            Reporter: Shaun Sabo
>            Assignee: Ishan Chattopadhyaya
>            Priority: Major
>         Attachments: SOLR-11815.patch
>
>
> I am running a collection with a persistent high volume of writes. When the leader goes down and recovers, it joins as a replica and asks the new leader for the files to Sync. The isIndexStale check is finding that some files differ in size and checksum which forces a fullCopy. Since our indexes are rather large, a rolling restart is resulting in large amounts of data transfer, and in some cases disk space contention issues.
> I do not believe the fullCopy is necessary given the circumstances. 
> Repro Steps:
> 1. collection/shard with 1 leader and 1 replica are accepting writes
>     - Pull interval is 30 seconds
>     - Hard Commit interval is 60 seconds
> 2. Replica executes an index pull and completes. 
> 3. Leader process Hard Commits (replica index is delayed)
> 4. leader process is killed (SIGTERM)
> 5. Replica takes over as new leader
> 6. New leader applies TLOG since last pull (cores are binary-divergent now)
> 7. Former leader comes back as New Replica
> 8. New replica initiates recovery
>     - Recovery detects that the generation and version are behind and a check is necessary
> 9. isIndexStale() detects that a segment exists on both the New Replica and New Leader but that the size and checksum differ. 
>     - This triggers fullCopy to be flagged on
> 10. Entirety of index is pulled regardless of changes
> The majority of files should not have changes, but everything gets pulled because of the first file it finds with a mismatched checksum. 
> Relevant Code:
> https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/handler/IndexFetcher.java#L516-L518
> https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/handler/IndexFetcher.java#L1105-L1126



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