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From "Devaraj Das (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1014) map/reduce is corrupting data between map and reduce
Date Thu, 15 Feb 2007 11:04:05 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-1014?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Devaraj Das updated HADOOP-1014:

    Attachment: zero-size-inmem-fs.patch

Riccardo, the problem with your testcase was that in the "readFields" method of the WritableWrapper
class you were not setting the "type" field so that "write" would always write the value '0'
for 'type' in the map output file. Hence the reduces won't get the intended map outputs. I
have attached the fixed TestMapRed.java.
In your map method, you instantiate a WritableWrapper object which has the type field correctly
set. After that you do output.collect which would have different behaviors in the versions
0.9 and 0.10+. In the version 0.9, the output.collect will write the data directly to the
final map output file. However, with the 0.10+ versions, the output gets buffered and written
later and there is deserialization/serialization happening when the output is finally written
to disk from the buffer. In your code, the deserialization code (readFields) was not setting
the type field and hence the serialization (write) would always write 0 (type - INT_WRITABLE)
for the type field. The reducer, thus, would never see UTF8.

Also attached a patch that would disable inmem merge (basically sets the buffer size for the
ramfs to 0, and does some checks for that). This should remove the blocker.

Mike, Albert and Riccardo - please comment whether this solves the issues you reported for
the time being. I will continue to debug the inmem merge. Must be some race condition somewhere
since the failures are not consistent. Thanks.

> map/reduce is corrupting data between map and reduce
> ----------------------------------------------------
>                 Key: HADOOP-1014
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1014
>             Project: Hadoop
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.11.1
>            Reporter: Owen O'Malley
>         Assigned To: Devaraj Das
>            Priority: Blocker
>             Fix For: 0.11.2
>         Attachments: TestMapRed.java, TestMapRed.patch, TestMapRed2.patch, zero-size-inmem-fs.patch
> It appears that a random data corruption is happening between the map and the reduce.
This looks to be a blocker until it is resolved. There were two relevant messages on hadoop-dev:
> from Mike Smith:
> The map/reduce jobs are not consistent in hadoop 0.11 release and trunk both
> when you rerun the same job. I have observed this inconsistency of the map
> output in different jobs. A simple test to double check is to use hadoop
> 0.11 with nutch trunk.
> from Albert Chern:
> I am having the same problem with my own map reduce jobs.  I have a job
> which requires two pieces of data per key, and just as a sanity check I make
> sure that it gets both in the reducer, but sometimes it doesn't.  What's
> even stranger is, the same tasks that complain about missing key/value pairs
> will maybe fail two or three times, but then succeed on a subsequent try,
> which leads me to believe that the bug has to do with randomization (I'm not
> sure, but I think the map outputs are shuffled?).
> All of my code works perfectly with 0.9, so I went back and just compared
> the sizes of the outputs.  For some jobs, the outputs from 0.11 were
> consistently 4 bytes larger, probably due to changes in SequenceFile.  But
> for others, the output sizes were all over the place.  Some partitions were
> empty, some were correct, and some were missing data.  There seems to be
> something seriously wrong with 0.11, so I suggest you use 0.9.  I've been
> trying to pinpoint the bug but its random nature is really annoying.

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