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From <Zlatin.Balev...@barclayscapital.com>
Subject RE: Exponential performance decay - mystery solved
Date Thu, 21 Jan 2010 19:41:13 GMT


Alright, the problem was caused by me setting the frequency of a block
report to 30 seconds.  The idea behind that was to create more load on
the Namenode, but I didn't notice that those block reports were taking
increasing amounts of time to generate.  During that time, a lock was
held which I'm guessing didn't allow the reporting datanode to perform
its functions.

On my hardware, with 100,000 blocks the report takes over 7 seconds.  So
every datanode was unavailable for 7 out of every 30 seconds.  Changing
the interval to a more reasonable value restored the insertion speed to
linear.

Apologies for creating this confusion, nevertheless it was a useful
thing to learn.

Regards,
Zlatin

-----Original Message-----
From: Eli Collins [mailto:eli@cloudera.com] 
Sent: Thursday, January 21, 2010 2:02 PM
To: hdfs-user@hadoop.apache.org
Subject: Re: Exponential performance decay - possible lead

>
> The messages are of the following:
>
> 2010-01-18 14:51:25,694 WARN org.apache.hadoop.hdfs.StateChange: 
> BLOCK* NameSystem.addStoredBlock: Redundant addStoredBlock request 
> received for blk_-5804440919363539694_1026 on ip.removed:port.removed 
> size 1024

This is odd, you should't be getting this warning, I don't see it when
running your benchmark on my cluster. Are there other relevant/warnings
errors in the NN or DN logs?

Thanks,
Eli
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