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https://issues.apache.org/jira/browse/SOLR-11815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16310479#comment-16310479
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John Gallagher commented on SOLR-11815:
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>From my uninformed view, it doesn't seem like the `isIndexStale` check is
>necessary.
If remove the check:
https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/handler/IndexFetcher.java#L516-L518
Later on in downloadIndexFiles:
https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/handler/IndexFetcher.java#L992
It will download any of those files if
1) checksum/size mismatch
2) fullCopyNeeded (called downloadCompleteIndex here)
3) alwaysDownload
So it seems like the {{isIndexStale}} check was forcing condition (2) to be
true, but actually just having condition (1) is probably sufficient, and
operates on a file-by-file basis.
It doesn't seem like removing that check alters the {{filesToDownload}} set in
any way, and it seems like {{downloadIndexFiles}} and {{isIndexStale}} use the
same {{compareFile}} check.
I was able to repro using ishan's script, though I had to make a minor
adjustment to the collection create:
https://gist.github.com/slackhappy/53f5674f2fcba434145b5db5adb07810
> 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
> Security Level: Public(Default Security Level. Issues are Public)
> Components: replication (java)
> Affects Versions: 7.2
> Environment: Oracle JDK 1.8
> Ubuntu 16.04
> Reporter: Shaun Sabo
> Assignee: Ishan Chattopadhyaya
>
> 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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