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From Dhruba Borthakur <dhr...@gmail.com>
Subject Re: Tracking Replication errors
Date Thu, 10 Sep 2009 04:35:42 GMT
The sender datanode sends the crc along with the data. This allows the
receiver datanode to detect corrupt data. The orignal crc was created by the
client that created the data in the block for the first time. The crc is not
kept in the namenode. To facilitate random access, there is a crc per 512
bytes of data... this is too much of metadata for the NN to hold in memory.

dhruba


On Wed, Sep 9, 2009 at 8:33 PM, Brian Bockelman <bbockelm@cse.unl.edu>wrote:

>
> 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?
>
> Brian
>
>
>  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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