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From Uma Maheswara Rao G <mahesw...@huawei.com>
Subject RE: Blocks are getting corrupted under very high load
Date Thu, 24 Nov 2011 02:21:54 GMT
Thanks Todd.

Finally we also started suspecting in that angle. Planned to take the file details before
reboot and after reboot.
With the above analysis i can confirm, whether the same issue or not.

One more thing to notice is that the difference between reboot time and last replica finalization
is ~1hr in some cases. 
Since the machine is rebooted due to kernal.hung_task_timeout_secs , in OS also that particular
thread might not got the chance to sync the data.

great one, HDFS-1539, I have merged all the bugs. Since this is an improvement, issue might
not come to my list :( .

Also found some OS level configs to do the filesystem operations synchronously 
    All directory updates within the filesystem should be done synchronously. This affects
the following system calls: creat, link, unlink, symlink, mkdir, rmdir, mknod and rename.

We suspected mainly the rename operation lost after reboot. Since metafile , blockfile rename
should happen when finalizing the block from BBW to current. ( at least not considered blocksize).

Anyway, thanks a lot for your great & valuable  time  with us here. After checking the
above OS logs, i will have a run with HDFS-1539.


From: Todd Lipcon [todd@cloudera.com]
Sent: Thursday, November 24, 2011 5:07 AM
To: common-dev@hadoop.apache.org
Cc: hdfs-dev@hadoop.apache.org
Subject: Re: Blocks are getting corrupted under very high load

On Wed, Nov 23, 2011 at 1:23 AM, Uma Maheswara Rao G
<maheswara@huawei.com> wrote:
> Yes, Todd,  block after restart is small and  genstamp also lesser.
>   Here complete machine reboot happend. The boards are configured like, if it is not
getting any CPU cycles  for 480secs, it will reboot himself.
>  kernal.hung_task_timeout_secs = 480 sec.

So sounds like the following happened:
- while writing file, the pipeline got reduced down to 1 node due to
timeouts from the other two
- soon thereafter (before more replicas were made), that last replica
kernel-paniced without syncing the data
- on reboot, the filesystem lost some edits from its ext3 journal, and
the block got moved back into the RBW directly, with truncated data
- hdfs did "the right thing" - at least what the algorithms say it
should do, because it had gotten a commitment for a later replica

If you have a build which includes HDFS-1539, you could consider
setting dfs.datanode.synconclose to true, which would have prevented
this problem.

Todd Lipcon
Software Engineer, Cloudera

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