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From "John Gallagher (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-11815) TLOG leaders going down and rejoining as a replica do fullCopy when not needed
Date Wed, 03 Jan 2018 23:36:00 GMT

    [ https://issues.apache.org/jira/browse/SOLR-11815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16310479#comment-16310479
] 

John Gallagher commented on SOLR-11815:
---------------------------------------

>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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