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https://issues.apache.org/jira/browse/HDFS-17960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18103872#comment-18103872
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ASF GitHub Bot commented on HDFS-17960:
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JHSUYU commented on PR #8672:
URL: https://github.com/apache/hadoop/pull/8672#issuecomment-5260673989
Hi @pan3793, could you take a look when you are available? Thanks very much!
Happy to revise it based on any feedback
> Snapshot count becomes inconsistent when replaying ordered snapshot deletion
> edits
> -----------------------------------------------------------------------------------
>
> Key: HDFS-17960
> URL: https://issues.apache.org/jira/browse/HDFS-17960
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: snapshots
> Affects Versions: 3.5.0
> Reporter: ZhenyuLi
> Priority: Major
> Labels: pull-request-available
>
> When edit logs generated with ordered snapshot deletion enabled are replayed
> with dfs.namenode.snapshot.deletion.ordered=false,
> SnapshotManager.numSnapshots may be decremented even though no snapshot is
> removed.
> HDFS-15590 added a replay-tolerance path in
> DirectorySnapshottableFeature.removeSnapshot(). If the requested snapshot
> does not exist, ordered deletion is disabled, and the namespace image is
> still being loaded, removeSnapshot() returns null:
> {code:java}
> if (!snapshotManager.isSnapshotDeletionOrdered()
> && !snapshotManager.isImageLoaded()) {
> return null;
> }
> {code}
> However, SnapshotManager.deleteSnapshot() ignores the return value and
> unconditionally decrements numSnapshots:
> {code:java}
> srcRoot.removeSnapshot(reclaimContext, snapshotName, now, this);
> numSnapshots.getAndDecrement();
> {code}
> Therefore, a tolerated no-op edit changes numSnapshots without changing the
> authoritative snapshotsByNames lists.
> h3. Proposed fix
> Use the return value of removeSnapshot() and decrement numSnapshots only
> when
> a Snapshot was actually removed:
> {code:java}
> final Snapshot removed = srcRoot.removeSnapshot(
> reclaimContext, snapshotName, now, this);
> if (removed != null) {
> numSnapshots.decrementAndGet();
> }
> {code}
> The resulting snapshot count inconsistency causes subsequent checkpoint or
> saveNamespace operations to fail at Preconditions.checkState(i ==
> sm.getNumSnapshots()) in
> FSImageFormatPBSnapshot.Saver.serializeSnapshotSection().
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