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https://issues.apache.org/jira/browse/HDFS-15415?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17141119#comment-17141119
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Stephen O'Donnell commented on HDFS-15415:
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The 002 patch removes the lock from all the logic except when calling
`getSortedFinalizedBlocks()`.
> Reduce locking in Datanode DirectoryScanner
> -------------------------------------------
>
> Key: HDFS-15415
> URL: https://issues.apache.org/jira/browse/HDFS-15415
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 3.4.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: HDFS-15415.001.patch, HDFS-15415.002.patch
>
>
> In HDFS-15406, we have a small change to greatly reduce the runtime and
> locking time of the datanode DirectoryScanner. They may be room for further
> improvement here:
> 1. These lines of code in DirectoryScanner#scan(), obtain a snapshot of the
> finalized blocks from memory, and then sort them, under the DN lock. However
> the blocks are stored in a sorted structure (FoldedTreeSet) and hence the
> sort should be unnecessary.
> {code}
> final List<ReplicaInfo> bl = dataset.getFinalizedBlocks(bpid);
> Collections.sort(bl); // Sort based on blockId
> {code}
> 2. From the scan step, we have captured a snapshot of what is on disk. After
> calling `dataset.getFinalizedBlocks(bpid);` as above we have taken a snapshot
> of in memory. The two snapshots are never 100% in sync as things are always
> changing as the disk is scanned.
> We are only comparing finalized blocks, so they should not really change:
> * If a block is deleted after our snapshot, our snapshot will not see it and
> that is OK.
> * A finalized block could be appended. If that happens both the genstamp and
> length will change, but that should be handled by reconcile when it calls
> `FSDatasetImpl.checkAndUpdate()`, and there is nothing stopping blocks being
> appended after they have been scanned from disk, but before they have been
> compared with memory.
> My suspicion is that we can do all the comparison work outside of the lock
> and checkAndUpdate() re-checks any differences later under the lock on a
> block by block basis.
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