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RE: a question on NameNode
Thank you Kai.. One more question please.

Does MapReduce run tasks of redundant blocks ?

Say you have only 1 block of data replicated 3 times, one block over each of three DNodes, block 1 - DN1 / block 1(replica #1) - DN2 / block1 (replica #2) - DN3

Will MR attempt:
a.       to start 3 Map tasks (one per replicated block) end execute them all

b.      to start 3 Map tasks (one per replicated block) end drop the other two as soon as one of the three executed successfully

c.       will start only 1 Map task (for just one block avoiding all replicated ones) and will attempt to start (another one of the replicated blocks) when and only when the initially task running (say on DN1)failed


From: Kai Voigt [mailto:[EMAIL PROTECTED]]
Sent: Monday, November 19, 2012 10:01 AM
Subject: Re: a question on NameNode
Am 19.11.2012 um 15:43 schrieb "Kartashov, Andy" <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>>:
So, what if DN2 is down, i.e. it is not sending any blocks' report.  Then NN (I guess) will figure out that it has 2 blocks (3,4) that has no home and that (without replication) it has no way of reconstructing the file A.txt. It must spit the error then.

One major feature of HDFS is its redundancy. Blocks are stored more than once (three times by default), so chances are good that another DataNode will have that block and report it during the safe mode phase. So the file will be accessible.


Kai Voigt
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