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From Amar Kamat <ama...@yahoo-inc.com>
Subject Re: how to recover if master node goes down?
Date Sun, 03 Feb 2008 13:43:13 GMT
Ben Kucinich wrote:
> I am new to Hadoop. I want to know a few things.
> I have a Hadoop cluster of 1 master node and N - 1 slave nodes. I am putting
> files into the DFS. If one of the slave node goes down, the data is still
> accessible due to proper replication.
There are 2 masters in hadoop, one for DFS(namenode) and one for 
MapReduce(JobTracker). I guess your question is directed for DFS.
> What will happen if the master node goes down? Will some slave node take
> over as the master node automatically? Or all the data would become
> inaccessible?
The namenode periodically backs-up the file-system image on to the 
secondary namenode. The secondary namenode doesn't come up 
automatically. The data will not be lost since the fs-image is backed up 
but will become temporarily inaccessible.
> If all the data would become inacessible, what recovery action can be taken.
> Say, I boot the master node system again and run start-all.sh again, would
> it be enough to get things going fine again? Will I be able to access the
> old data from the DFS again?
You just need to log on to the secondary namenode and start the namenode 
daemon. Yes this would be enough to get the things started and you would 
be able to access the data again.

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