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From Arun C Murthy <...@hortonworks.com>
Subject Re: MapReduce jobs hanging or failing near completion
Date Tue, 02 Aug 2011 05:08:13 GMT
On Aug 1, 2011, at 9:47 PM, Kai Ju Liu wrote:

> Hi Arun. Since migrating HDFS off EBS-mounted volumes and onto ephemeral disks, the problem
has actually persisted. Now, however, there is no evidence of errors on any of the mappers.
The job tracker lists one less map completed than the map total, while the job details show
all mappers as having completed. The jobs "hang" in this state as before.

Are any of your job's reducers completing? Do you see 'fetch failures' messages either in
JT logs or reducers' (tasks) logs?

If not it's clear that the JobTracker/Scheduler (which Scheduler are you using btw?) are 'losing'
tasks which is a serious bug. You say that you are running CDH - unfortunately I have no idea
what patchsets you run with it. I can't, at the top of my head, remember the JT/CapacityScheduler
losing a task - but I maintained Yahoo clusters which ran hadoop-0.20.203.

Here is something worth trying: 
$ cat JOBTRACKER.log | grep Assigning | grep _<clustertimestamp>_<jobid>_m_*

The JOBTRACKER.log is the JT's log file on the JT host and if your jobid is job_12345342432_0001,
then <clustertimestamp> == 12345342432 and <jobid> == 0001.

Good luck.


> Is there something in particular I should be looking for on my local disks? Hadoop fsck
shows all clear, but I'll have to wait until morning to take individual nodes offline to check
their disks. Any further details you might have would be very helpful. Thanks!
> Kai Ju
> On Tue, Jul 19, 2011 at 1:50 PM, Arun C Murthy <acm@hortonworks.com> wrote:
> Is this reproducible? If so, I'd urge you to check your local disks...
> Arun
> On Jul 19, 2011, at 12:41 PM, Kai Ju Liu wrote:
>> Hi Marcos. The issue appears to be the following. A reduce task is unable to fetch
results from a map task on HDFS. The map task is re-run, but the map task is now unable to
retrieve information that it needs to run. Here is the error from the second map task:
>> java.io.FileNotFoundException: /mnt/hadoop/mapred/local/taskTracker/hadoop/jobcache/job_201107171642_0560/attempt_201107171642_0560_m_000292_1/output/spill0.out
>> 	at org.apache.hadoop.fs.RawLocalFileSystem.open(RawLocalFileSystem.java:176)
>> 	at org.apache.hadoop.fs.FileSystem.open(FileSystem.java:456)
>> 	at org.apache.hadoop.mapred.Merger$Segment.init(Merger.java:205)
>> 	at org.apache.hadoop.mapred.Merger$Segment.access$100(Merger.java:165)
>> 	at org.apache.hadoop.mapred.Merger$MergeQueue.merge(Merger.java:418)
>> 	at org.apache.hadoop.mapred.Merger$MergeQueue.merge(Merger.java:381)
>> 	at org.apache.hadoop.mapred.Merger.merge(Merger.java:77)
>> 	at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.mergeParts(MapTask.java:1547)
>> 	at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.flush(MapTask.java:1179)
>> 	at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:391)
>> 	at org.apache.hadoop.mapred.MapTask.run(MapTask.java:324)
>> 	at org.apache.hadoop.mapred.Child$4.run(Child.java:268)
>> 	at java.security.AccessController.doPrivileged(Native Method)
>> 	at javax.security.auth.Subject.doAs(Subject.java:396)
>> 	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1115)
>> 	at org.apache.hadoop.mapred.Child.main(Child.java:262)
>> I have been having general difficulties with HDFS on EBS, which pointed me in this
direction. Does this sound like a possible hypothesis to you? Thanks!
>> Kai Ju
>> P.S. I am migrating off of HDFS on EBS, so I will post back with further results
as soon as I have them.
>> On Thu, Jul 7, 2011 at 6:36 PM, Marcos Ortiz <mlortiz@uci.cu> wrote:
>> El 7/7/2011 8:43 PM, Kai Ju Liu escribió:
>> Over the past week or two, I've run into an issue where MapReduce jobs
>> hang or fail near completion. The percent completion of both map and
>> reduce tasks is often reported as 100%, but the actual number of
>> completed tasks is less than the total number. It appears that either
>> tasks backtrack and need to be restarted or the last few reduce tasks
>> hang interminably on the copy step.
>> In certain cases, the jobs actually complete. In other cases, I can't
>> wait long enough and have to kill the job manually.
>> My Hadoop cluster is hosted in EC2 on instances of type c1.xlarge with 4
>> attached EBS volumes. The instances run Ubuntu 10.04.1 with the
>> 2.6.32-309-ec2 kernel, and I'm currently using Cloudera's CDH3u0
>> distribution. Has anyone experienced similar behavior in their clusters,
>> and if so, had any luck resolving it? Thanks!
>> Can you post here your NN and DN logs files?
>> Regards
>> Kai Ju
>> -- 
>> Marcos Luís Ortíz Valmaseda
>>  Software Engineer (UCI)
>>  Linux User # 418229
>>  http://marcosluis2186.posterous.com
>>  http://twitter.com/marcosluis2186

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