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https://issues.apache.org/jira/browse/HDFS-15406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17136570#comment-17136570
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Stephen O'Donnell commented on HDFS-15406:
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Thinking about this some more, as `dataset.getFinalizedBlocks(bpid);` makes a
new copy of all the finalized blocks in the block pool, do we even need to hold
the DN lock while we compare the differences between on disk and in memory?
From the scan step, we have captured a snapshot of what is on disk. After
calling `dataset.getFinalizedBlocks(bpid);` 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.
I am not 100% sure about this, but my suspicion is that we can do a lot of this
work outside of the lock ad checkAndUpdate() re-checks any differences later
under the lock on a block by block basis.
> Improve the speed of Datanode Block Scan
> ----------------------------------------
>
> Key: HDFS-15406
> URL: https://issues.apache.org/jira/browse/HDFS-15406
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: hemanthboyina
> Assignee: hemanthboyina
> Priority: Major
> Attachments: HDFS-15406.001.patch
>
>
> In our customer cluster we have approx 10M blocks in one datanode
> the Datanode to scans all the blocks , it has taken nearly 5mins
> {code:java}
> 2020-06-10 12:17:06,869 | INFO |
> java.util.concurrent.ThreadPoolExecutor$Worker@3b4bea70[State = -1, empty
> queue] | BlockPool BP-1104115233-**.**.**.**-1571300215588 Total blocks:
> 11149530, missing metadata files:472, missing block files:472, missing blocks
> in memory:0, mismatched blocks:0 | DirectoryScanner.java:473
> 2020-06-10 12:17:06,869 | WARN |
> java.util.concurrent.ThreadPoolExecutor$Worker@3b4bea70[State = -1, empty
> queue] | Lock held time above threshold: lock identifier:
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl
> lockHeldTimeMs=329854 ms. Suppressed 0 lock warnings. The stack trace is:
> java.lang.Thread.getStackTrace(Thread.java:1559)
> org.apache.hadoop.util.StringUtils.getStackTrace(StringUtils.java:1032)
> org.apache.hadoop.util.InstrumentedLock.logWarning(InstrumentedLock.java:148)
> org.apache.hadoop.util.InstrumentedLock.check(InstrumentedLock.java:186)
> org.apache.hadoop.util.InstrumentedLock.unlock(InstrumentedLock.java:133)
> org.apache.hadoop.util.AutoCloseableLock.release(AutoCloseableLock.java:84)
> org.apache.hadoop.util.AutoCloseableLock.close(AutoCloseableLock.java:96)
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.scan(DirectoryScanner.java:475)
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.reconcile(DirectoryScanner.java:375)
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.run(DirectoryScanner.java:320)
> java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
> java.lang.Thread.run(Thread.java:748)
> | InstrumentedLock.java:143 {code}
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