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Date: Mon, 19 Oct 2009 22:49:59 +0000 (UTC)
From: "Todd Lipcon (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-127) DFSClient block read failures cause
 open DFSInputStream to become unusable
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    [ https://issues.apache.org/jira/browse/HDFS-127?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12767593#action_12767593 ] 

Todd Lipcon commented on HDFS-127:
----------------------------------

bq. By default, "dfs.client.max.block.acquire.failures" is 3. If more than 3 replicas, we could fail though good replicas out on cluster. Should DFSClient set this.maxBlockAcquireFailures default to max of dfs.replication/dfs.client.max.block.acquire.failures?

Another example of bad naming - this variable actually refers to the number of times the read path will go back to the namenode for a new set of locations. It's to deal with the case when some locations have been cached for some amount of time, during which the blocks have been moved to new locations.

> DFSClient block read failures cause open DFSInputStream to become unusable
> --------------------------------------------------------------------------
>
>                 Key: HDFS-127
>                 URL: https://issues.apache.org/jira/browse/HDFS-127
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs client
>            Reporter: Igor Bolotin
>         Attachments: 4681.patch, h127_20091016.patch, h127_20091019.patch
>
>
> We are using some Lucene indexes directly from HDFS and for quite long time we were using Hadoop version 0.15.3.
> When tried to upgrade to Hadoop 0.19 - index searches started to fail with exceptions like:
> 2008-11-13 16:50:20,314 WARN [Listener-4] [] DFSClient : DFS Read: java.io.IOException: Could not obtain block: blk_5604690829708125511_15489 file=/usr/collarity/data/urls-new/part-00000/20081110-163426/_0.tis
> at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.chooseDataNode(DFSClient.java:1708)
> at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.blockSeekTo(DFSClient.java:1536)
> at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.read(DFSClient.java:1663)
> at java.io.DataInputStream.read(DataInputStream.java:132)
> at org.apache.nutch.indexer.FsDirectory$DfsIndexInput.readInternal(FsDirectory.java:174)
> at org.apache.lucene.store.BufferedIndexInput.refill(BufferedIndexInput.java:152)
> at org.apache.lucene.store.BufferedIndexInput.readByte(BufferedIndexInput.java:38)
> at org.apache.lucene.store.IndexInput.readVInt(IndexInput.java:76)
> at org.apache.lucene.index.TermBuffer.read(TermBuffer.java:63)
> at org.apache.lucene.index.SegmentTermEnum.next(SegmentTermEnum.java:131)
> at org.apache.lucene.index.SegmentTermEnum.scanTo(SegmentTermEnum.java:162)
> at org.apache.lucene.index.TermInfosReader.scanEnum(TermInfosReader.java:223)
> at org.apache.lucene.index.TermInfosReader.get(TermInfosReader.java:217)
> at org.apache.lucene.index.SegmentTermDocs.seek(SegmentTermDocs.java:54) 
> ...
> The investigation showed that the root of this issue is that we exceeded # of xcievers in the data nodes and that was fixed by changing configuration settings to 2k.
> However - one thing that bothered me was that even after datanodes recovered from overload and most of client servers had been shut down - we still observed errors in the logs of running servers.
> Further investigation showed that fix for HADOOP-1911 introduced another problem - the DFSInputStream instance might become unusable once number of failures over lifetime of this instance exceeds configured threshold.
> The fix for this specific issue seems to be trivial - just reset failure counter before reading next block (patch will be attached shortly).
> This seems to be also related to HADOOP-3185, but I'm not sure I really understand necessity of keeping track of failed block accesses in the DFS client.

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