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https://issues.apache.org/jira/browse/HBASE-21387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16678531#comment-16678531
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Ted Yu commented on HBASE-21387:
--------------------------------
Here is a brief summary of the approaches I tried, with most recent first -
which is expected to be reviewed:
21387.v9.txt :
At the beginning of getUnreferencedFiles, snapshot is temporarily disabled.
We check whether there is in-flight snapshot. If there is, don't list any file
as unreferenced.
Otherwise, fill out unreferenced files. During this time, snapshot attempt
would be declined.
At the end of getUnreferencedFiles, snapshot is enabled.
two-pass-cleaner.v9.txt :
Cleaner chore stores candidates from previous invocation of the chore. The
chore would calculate the intersection of previous candidates and current
candidates.
The downside of this approach is that the extra candidates from previous
iteration consumes (potentially large) memory.
21387.v8.txt :
SnapshotFileCache would try to obtain in progress snapshot under the lock.
However, since the timing of when in progress snapshot completes is not under
the control of SnapshotFileCache, it is hard to avoid race condition.
> 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, 21387.v9.txt, two-pass-cleaner.v4.txt,
> two-pass-cleaner.v6.txt, two-pass-cleaner.v9.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.
> Here is timeline given by Josh illustrating the scenario:
> At time T0, we are checking if F1 is referenced. At time T1, there is a
> snapshot S1 in progress that is referencing a file F1. refreshCache() is
> called, but no completed snapshot references F1. At T2, the snapshot S1,
> which references F1, completes. At T3, we check in-progress snapshots and S1
> is not included. Thus, F1 is marked as unreferenced even though S1 references
> it.
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