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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3831) slow-reading dfs clients do not recover from datanode-write-timeouts
Date Fri, 25 Jul 2008 18:55:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3831?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12617004#action_12617004

Raghu Angadi commented on HADOOP-3831:

Before 0.17, DataNode did not close client socket on its own. So when DFSClient detects any
socket error from a datanode, it marks it as dead and the list of 'deadnodes' is maintained
for each open file. This persists through life of the the open file. So the deadnode will
not be contacted even for other blocks. This policy will not work with 0.17 when there are
slow clients. 
Apart from slow clients where DataNode closes the connection after an 8 minute write timeout,
HADOOP-3633 in 0.17.2 introduced another (I think more likely to happen) case where client
sees errors from a datanode : if it already has 256 transfers going on.

Couple of fairly simple fixes :

* 1. When client detects a connection failure after it read some bytes from a DataNode, it
should just retry again with same datanode before moving on to next one. 
         *- This will fix the write-timeout problem as reported in this jira.
         *- Checksum errors will still be handled the same way as before.

* 2. Clear the 'deadnode' list when client moves to a new block.
         *- This will reduce the effect of HADOOP-3633 when there are a lot of clients.
         *- the larger issue  remains : When is a datanode really dead and when should it
be retried after transient errors?

This jira mainly requires (1). We could postpone (2) until we get more experience with HADOOP-3633.

> slow-reading dfs clients do not recover from datanode-write-timeouts
> --------------------------------------------------------------------
>                 Key: HADOOP-3831
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3831
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.17.1
>            Reporter: Christian Kunz
>            Assignee: Raghu Angadi
> Some of our applications read through certain files from dfs (using libhdfs) much slower
than through others, such that they trigger the write timeout introduced in 0.17.x into the
datanodes. Eventually they fail.
> Dfs clients should be able to recover from such a situation.
> In the meantime, would setting
> dfs.datanode.socket.write.timeout=0
> in hadoop-site.xml help?
> Here are the exceptions I see:
> DataNode:
> 2008-07-24 00:12:40,167 WARN org.apache.hadoop.dfs.DataNode: xxx:50010:Got exception
while serving blk_3304550638094049
> 753 to /yyy:
> java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be
ready for write. ch : java.nio.channels.
> SocketChannel[connected local=/xxx:50010 remote=/yyy:42542]
>         at org.apache.hadoop.net.SocketIOWithTimeout.doIO(SocketIOWithTimeout.java:170)
>         at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:144)
>         at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:105)
>         at java.io.BufferedOutputStream.write(BufferedOutputStream.java:105) 
>         at java.io.DataOutputStream.write(DataOutputStream.java:90)
>         at org.apache.hadoop.dfs.DataNode$BlockSender.sendChunks(DataNode.java:1774)
>         at org.apache.hadoop.dfs.DataNode$BlockSender.sendBlock(DataNode.java:1813)
>         at org.apache.hadoop.dfs.DataNode$DataXceiver.readBlock(DataNode.java:1039) 
>         at org.apache.hadoop.dfs.DataNode$DataXceiver.run(DataNode.java:968)
>         at java.lang.Thread.run(Thread.java:619)
> DFS Client:
> 08/07/24 00:13:28 WARN dfs.DFSClient: Exception while reading from blk_3304550638094049753
of zzz from xxx:50010: java.io.IOException: Premeture EOF from inputStream
>     at org.apache.hadoop.io.IOUtils.readFully(IOUtils.java:100)
>     at org.apache.hadoop.dfs.DFSClient$BlockReader.readChunk(DFSClient.java:967)
>     at org.apache.hadoop.fs.FSInputChecker.readChecksumChunk(FSInputChecker.java:236)
>     at org.apache.hadoop.fs.FSInputChecker.read1(FSInputChecker.java:191)
>     at org.apache.hadoop.fs.FSInputChecker.read(FSInputChecker.java:159)
>     at org.apache.hadoop.dfs.DFSClient$BlockReader.read(DFSClient.java:829)
>     at org.apache.hadoop.dfs.DFSClient$DFSInputStream.readBuffer(DFSClient.java:1352)
>     at org.apache.hadoop.dfs.DFSClient$DFSInputStream.read(DFSClient.java:1388)
>     at java.io.DataInputStream.read(DataInputStream.java:83)
> 08/07/24 00:13:28 INFO dfs.DFSClient: Could not obtain block blk_3304550638094049753
from any node:  java.io.IOException: No live nodes contain current block

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