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From Brian Bockelman <bbock...@cse.unl.edu>
Subject Re: Tracking Replication errors
Date Thu, 10 Sep 2009 03:33:53 GMT

On Sep 9, 2009, at 10:25 PM, Dhruba Borthakur wrote:

> when a block is being received by a datanode (either because of a
> replication request or from a client write), the datanode verifies  
> crc.

Ah, so I'm wrong and the answer is better than I expected.  Never have  
I been so happy to be wrong :)

Where is the "master" crc kept?  The sending datanode?  I assume this  
means that the first datanode to write the block is the "master".

It's late, but I vaguely remember that 0.21.0 does some elaborate  
gymnastics to determine who has the master copy of the block.

Is the CRC kept in the NN?  Any specific reason why not, beyond  
decreasing the memory footprint?


> Also, the there is a thread in the datanode that periodically  
> verifies crc
> of existing blocks.
> dhruba
> On Wed, Sep 9, 2009 at 7:27 PM, Brian Bockelman  
> <bbockelm@cse.unl.edu>wrote:
>> Hey everyone,
>> We're going through a review of our usage of HDFS (it's a good  
>> thing! -
>> we're trying to get "official").  One reviewer asked a good  
>> question that I
>> don't know the answer too - could you help?  To quote,
>> "What steps do you take to ensure the block rebalancing produces
>> non-corrupted files?  Do you have to wait 2 weeks before you  
>> discover this?"
>> I believe the correct answer is:
>> """
>> When a block is replicated from one node to another, only the  
>> resulting
>> block size is checked.  The checksums on the source and destination  
>> are not
>> compared.  Therefore, if there's any corruption that occurs, it  
>> would take
>> until the next block verification to detect it.
>> """
>> If you look at TCP error rates and random memory corruptions, it  
>> wouldn't
>> be surprising to see silent errors in copying between nodes,  
>> especially on
>> multi-hundred-TB or PB scale installs.
>> Any comments?
>> Brian

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