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From Harsh J <ha...@cloudera.com>
Subject Re: what happens when a datanode rejoins?
Date Tue, 11 Sep 2012 09:10:17 GMT


On Tue, Sep 11, 2012 at 2:36 PM, Mehul Choube <Mehul_Choube@symantec.com> wrote:
>> The namenode will asynchronously replicate the blocks to other datanodes
>> in order to maintain the replication factor after a datanode has not been in
>> contact for 10 minutes.
> What happens when the datanode rejoins after namenode has already
> re-replicated the blocs it was managing?

The block count total goes +1, and the file's block is treated as an
over-replicated one.

> Will namenode ask the datanode to discard the blocks and start managing new
> blocks?

Yes, this may happen.

> Or will namenode discard the new blocks which were replicated due to
> unavailability of this datanode?

It deletes extra blocks while still keeping the block placement policy
in mind. It may delete any block replica as long as the placement
policy is not violated by doing so.

Harsh J

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