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From Tianqiang Peter Li <peter...@gmail.com>
Subject Re: Nonempty files become empty after being saved to Hdfs.
Date Fri, 24 Feb 2012 07:31:00 GMT
Hi, Denny,
Thanks for the info, you are right about his, after some digging, I found
there were no data loss, the root cause of getting empty file is that
during the file rotation, the old file was closed, the new empty new file
was created/opened, then hdfs tried to create the small file(acting as
symbol link in hdfs) for this file, but for some reasons(gc?), the
operation for creating this symbol link file stuck for 2 minutes, then
right after it was successfully created, the scribe did a periodic check,
found this file has passed the rotation window(the rotation period is
1min), then rotate out this newly-created empty file. It's good to know
there is no data integrity issue. I am still looking at where this big
hiccup comes from. Anyway, thanks for the helpful comments on this.


On Sun, Feb 19, 2012 at 6:25 PM, Denny Ye <dennyy99@gmail.com> wrote:

> hi Peter,
>        I met this problem in our environment at one time. It may caused by
> the blank data stream from Scribe to HDFS. In fixed rotate model, there is
> no any data. Otherwise, NameNode can start the 'allocate block' operation
> during the first data packet to HDFS client. Also, this only one tip to
> your doubt.
> -Regards
> Denny Ye
> 2012/2/19 Tianqiang Peter Li <peterltq@gmail.com>
>> Hi, guys,
>> I am using scribe to write data directly to hdfs, it works well most of
>> the time, but sporadically, I found some of files(1-2 out of 6000 per day)
>> written to hdfs become empty files, interestingly, this only happens when
>> hdfs is busy handing lots of other write request at the same time, here is
>> log pulled from namenode's log output, it seems the allocateBlock operation
>> following the create operation is vanished, unlike other normal operations.
>> Here are the comparison of a normal file and a abnormal one below:
>> ----------- *Normal* one  (
>> file:/prefix-2012-02-18_00410 )---------------------------------
>> 2012-02-18 02:32:15,873 INFO
>> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.audit: ugi=user ip=/
>>        cmd=create      src=/prefix-2012-02-18_00410
>> dst=null        perm=user:supergroup:rw-r--r--
>> 2012-02-18 02:32:15,905 INFO org.apache.hadoop.hdfs.StateChange: *BLOCK*
>> NameSystem.allocateBlock*: /prefix-2012-02-18_00410.
>> blk_-2839230806711337016_129091175
>> 2012-02-18 02:32:17,422 INFO org.apache.hadoop.hdfs.StateChange: Removing
>> lease on  file /prefix-2012-02-18_00410 from client DFSClient_-36442571
>> 2012-02-18 02:32:17,422 INFO org.apache.hadoop.hdfs.StateChange: DIR*
>> NameSystem.completeFile: file /prefix-2012-02-18_00410 is closed by
>> DFSClient_-36442571
>> ----------- End --------------
>> ----------- *Problematic* one (
>> file:/prefix-2012-02-18_00409 )----------------------------
>> 2012-02-18 02:30:47,287 INFO
>> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.audit: ugi=user ip=/
>>        cmd=create      src=/prefix-2012-02-18_00409
>> dst=null        perm=user:supergroup:rw-r--r--
>> *<comment: no allocateBlock is found, and there is 60-90 sec gap between>
>> *
>> 2012-02-18 02:32:15,648 INFO org.apache.hadoop.hdfs.StateChange: Removing
>> lease on  file /prefix-2012-02-18_00409 from client DFSClient_748605466
>> 2012-02-18 02:32:15,648 INFO org.apache.hadoop.hdfs.StateChange: DIR*
>> NameSystem.completeFile: file /prefix-2012-02-18_00409 is closed by
>> DFSClient_748605466
>> ----------- End -------------------
>> Not sure this is more scribe-related or hdfs-related, let me try shooting
>> the question in this mailing-list.
>> The 90 secs delay looks suspicious to me, and might cause this issue as I
>> guess, did you guys meet this situation before? Any light shed on me about
>> why is that and how to work it around to guarantee the entire file has been
>> successfully saved into Hdfs?  Any places I should focus on for
>> improvements?(on hdfs side(e.g. hdfs tunning, gc tuning etc) or application
>> side(e.g. timeout setting on hdfs api calling in scribe), your
>> suggestions/thoughts are greatly appreciated.
>> Regards,
>> Peter

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