[jira] [Comment Edited] (HBASE-21387) Race condition surrounding in progress snapshot handling in snapshot cache leads to loss of snapshot files

["Ted Yu (JIRA)" <ji...@apache.org>]

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

Ted Yu edited comment on HBASE-21387 at 11/6/18 12:47 AM:
----------------------------------------------------------

In two-pass-cleaner.v6.txt , the reference to previous round is changed to Set<PathAndLength>.
The length of file is needed by HFileCleaner


was (Author: yuzhihong@gmail.com):
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.v6.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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