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https://issues.apache.org/jira/browse/HBASE-21387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16675900#comment-16675900
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Ted Yu commented on HBASE-21387:
--------------------------------

In two-pass-cleaner.v5.txt , the reference to previous round is changed to 
Set<Path>.

> Race condition surrounding in progress snapshot handling in snapshot cache 
> leads to loss of snapshot files
> ----------------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-21387
>                 URL: https://issues.apache.org/jira/browse/HBASE-21387
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Ted Yu
>            Assignee: Ted Yu
>            Priority: Major
>              Labels: snapshot
>         Attachments: 21387.dbg.txt, 21387.v2.txt, 21387.v3.txt, 21387.v6.txt, 
> 21387.v7.txt, 21387.v8.txt, two-pass-cleaner.v4.txt, two-pass-cleaner.v5.txt
>
>
> During recent report from customer where ExportSnapshot failed:
> {code}
> 2018-10-09 18:54:32,559 ERROR [VerifySnapshot-pool1-t2] 
> snapshot.SnapshotReferenceUtil: Can't find hfile: 
> 44f6c3c646e84de6a63fe30da4fcb3aa in the real 
> (hdfs://in.com:8020/apps/hbase/data/data/.../a/44f6c3c646e84de6a63fe30da4fcb3aa)
>  or archive 
> (hdfs://in.com:8020/apps/hbase/data/archive/data/.../a/44f6c3c646e84de6a63fe30da4fcb3aa)
>  directory for the primary table. 
> {code}
> We found the following in log:
> {code}
> 2018-10-09 18:54:23,675 DEBUG 
> [00:16000.activeMasterManager-HFileCleaner.large-1539035367427] 
> cleaner.HFileCleaner: Removing: 
> hdfs:///apps/hbase/data/archive/data/.../a/44f6c3c646e84de6a63fe30da4fcb3aa 
> from archive
> {code}
> The root cause is race condition surrounding in progress snapshot(s) handling 
> between refreshCache() and getUnreferencedFiles().
> There are two callers of refreshCache: one from RefreshCacheTask#run and the 
> other from SnapshotHFileCleaner.
> Let's look at the code of refreshCache:
> {code}
>       if (!name.equals(SnapshotDescriptionUtils.SNAPSHOT_TMP_DIR_NAME)) {
> {code}
> whose intention is to exclude in progress snapshot(s).
> Suppose when the RefreshCacheTask runs refreshCache, there is some in 
> progress snapshot (about to finish).
> When SnapshotHFileCleaner calls getUnreferencedFiles(), it sees that 
> lastModifiedTime is up to date. So cleaner proceeds to check in progress 
> snapshot(s). However, the snapshot has completed by that time, resulting in 
> some file(s) deemed unreferenced.



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