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From Raghu Angadi <rang...@yahoo-inc.com>
Subject Re: 0.18.1 datanode psuedo deadlock problem
Date Fri, 09 Jan 2009 22:45:47 GMT

The scan required for each block report is well known issue and it can 
be fixed. It was discussed multiple times (e.g. 


Earlier, inline 'du' on datanodes used to cause the same problem and 
they they were moved to a separate thread (HADOOP-3232). block reports 
can do the same...

Though 2M blocks on DN is very large, there is no reason block reports 
should break things. Once we fix block reports, something else might 
break.. but that is different issue.


Jason Venner wrote:
> The problem we are having is that datanodes periodically stall for 10-15 
> minutes and drop off the active list and then come back.
> What is going on is that a long operation set is holding the lock on on 
> FSDataset.volumes, and all of the other block service requests stall 
> behind this lock.
> "DataNode: [/data/dfs-video-18/dfs/data]" daemon prio=10 tid=0x4d7ad400 
> nid=0x7c40 runnable [0x4c698000..0x4c6990d0]
>   java.lang.Thread.State: RUNNABLE
>    at java.lang.String.lastIndexOf(String.java:1628)
>    at java.io.File.getName(File.java:399)
>    at 
> org.apache.hadoop.dfs.FSDataset$FSDir.getGenerationStampFromFile(FSDataset.java:148)

>    at 
> org.apache.hadoop.dfs.FSDataset$FSDir.getBlockInfo(FSDataset.java:181)
>    at 
> org.apache.hadoop.dfs.FSDataset$FSVolume.getBlockInfo(FSDataset.java:412)
>    at 
> org.apache.hadoop.dfs.FSDataset$FSVolumeSet.getBlockInfo(FSDataset.java:511) 
>    - locked <0x551e8d48> (a org.apache.hadoop.dfs.FSDataset$FSVolumeSet)
>    at org.apache.hadoop.dfs.FSDataset.getBlockReport(FSDataset.java:1053)
>    at org.apache.hadoop.dfs.DataNode.offerService(DataNode.java:708)
>    at org.apache.hadoop.dfs.DataNode.run(DataNode.java:2890)
>    at java.lang.Thread.run(Thread.java:619)
> This is basically taking a stat on every hdfs block on the datanode, 
> which in our case is ~ 2million, and can take 10+ minutes (we may be 
> experiencing problems with our raid controller but have no visibility 
> into it) at the OS level the file system seems fine and operations 
> eventually finish.
> It appears that a couple of different data structures are being locked 
> with the single object FSDataset$Volume.
> Then this happens:
> "org.apache.hadoop.dfs.DataNode$DataXceiver@1bcee17" daemon prio=10 
> tid=0x4da8d000 nid=0x7ae4 waiting for monitor entry 
> [0x459fe000..0x459ff0d0]
>   java.lang.Thread.State: BLOCKED (on object monitor)
>    at 
> org.apache.hadoop.dfs.FSDataset$FSVolumeSet.getNextVolume(FSDataset.java:473) 
>    - waiting to lock <0x551e8d48> (a 
> org.apache.hadoop.dfs.FSDataset$FSVolumeSet)
>    at org.apache.hadoop.dfs.FSDataset.writeToBlock(FSDataset.java:934)
>    - locked <0x54e550e0> (a org.apache.hadoop.dfs.FSDataset)
>    at 
> org.apache.hadoop.dfs.DataNode$BlockReceiver.<init>(DataNode.java:2322)
>    at 
> org.apache.hadoop.dfs.DataNode$DataXceiver.writeBlock(DataNode.java:1187)
>    at org.apache.hadoop.dfs.DataNode$DataXceiver.run(DataNode.java:1045)
>    at java.lang.Thread.run(Thread.java:619)
> which locks the FSDataset while waiting on the volume object
> and now all of the Datanode operations stall waiting on the FSDataset 
> object.
> ----------
> Our particular installation doesn't use multiple directories for hdfs, 
> so a first simple hack for a local fix would be to modify getNextVolume 
> to just return the single volume and not be synchronized
> A richer alternative would be to make the locking more fine grained on 
> FSDataset$FSVolumeSet.
> Of course we are also trying to fix the file system performance and dfs 
> block loading that results in the block report taking a long time.
> Any suggestions or warnings?
> Thanks.

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