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MapReduce, mail # user - NameNode failure and recovery!


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NameNode failure and recovery!
Rahul Bhattacharjee 2013-04-03, 14:40
Hi all,

I was reading about Hadoop and got to know that there are two ways to
protect against the name node failures.

1) To write to a nfs mount along with the usual local disk.
 -or-
2) Use secondary name node. In case of failure of NN , the SNN can take in
charge.

My questions :-

1) SNN is always lagging , so when SNN becomes primary in event of a NN
failure ,  then the edits which have not been merged into the image file
would be lost , so the system of SNN would not be consistent with the NN
before its failure.

2) Also I have read that other purpose of SNN is to periodically merge the
edit logs with the image file. In case a setup goes with option #1 (writing
to NFS, no SNN) , then who does this merging.

Thanks,
Rahul