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https://issues.apache.org/jira/browse/HDFS-16438?focusedWorklogId=716166&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-716166
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ASF GitHub Bot logged work on HDFS-16438:
-----------------------------------------
Author: ASF GitHub Bot
Created on: 27/Jan/22 02:47
Start Date: 27/Jan/22 02:47
Worklog Time Spent: 10m
Work Description: tomscut commented on pull request #3928:
URL: https://github.com/apache/hadoop/pull/3928#issuecomment-1022794357
Thanks @sodonnel for your comments and detailed explanations.
I'm sorry I said the wrong number 5M, we have more than 10 million blocks
per node, and 20 disks, so more than 500,000 blocks per storage.
Your explanation makes sense to me. During `scanDatanodeStorage`, there is
`listRemove` behavior at the same time, so there will indeed be inconsistency.
Can we copy the blocks of each storage to a `list` first? This is a
lightweight operation. Then scan the `list` and release the lock once for part
of scan to reduce the lock holding time. After scan, clear the `list`. Do you
think this is feasible?
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Issue Time Tracking
-------------------
Worklog Id: (was: 716166)
Time Spent: 1.5h (was: 1h 20m)
> Avoid holding read locks for a long time when scanDatanodeStorage
> -----------------------------------------------------------------
>
> Key: HDFS-16438
> URL: https://issues.apache.org/jira/browse/HDFS-16438
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: tomscut
> Assignee: tomscut
> Priority: Major
> Labels: pull-request-available
> Attachments: image-2022-01-25-23-18-30-275.png
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> At the time of decommission, if use {*}DatanodeAdminBackoffMonitor{*}, there
> is a heavy operation: {*}scanDatanodeStorage{*}. If the number of blocks on a
> storage is large(more than 5 hundred thousand), and GC performance is also
> poor, it may hold *read lock* for a long time, we should optimize it.
> !image-2022-01-25-23-18-30-275.png|width=764,height=193!
> {code:java}
> 2021-12-22 07:49:01,279 INFO namenode.FSNamesystem
> (FSNamesystemLock.java:readUnlock(220)) - FSNamesystem scanDatanodeStorage
> read lock held for 5491 ms via
> java.lang.Thread.getStackTrace(Thread.java:1552)
> org.apache.hadoop.util.StringUtils.getStackTrace(StringUtils.java:1032)
> org.apache.hadoop.hdfs.server.namenode.FSNamesystemLock.readUnlock(FSNamesystemLock.java:222)
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.readUnlock(FSNamesystem.java:1641)
> org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor.scanDatanodeStorage(DatanodeAdminBackoffMonitor.java:646)
> org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor.checkForCompletedNodes(DatanodeAdminBackoffMonitor.java:417)
> org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor.check(DatanodeAdminBackoffMonitor.java:300)
> org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor.run(DatanodeAdminBackoffMonitor.java:201)
> 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:1142)
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
> java.lang.Thread.run(Thread.java:745)
> Number of suppressed read-lock reports: 0
> Longest read-lock held interval: 5491 {code}
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