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Re: Questions about BN and CN
The CheckpointNode creates checkpoints of the namespace, but does not keep
an up-to-date state of the namespace in memory.
If primary NN fails CheckpointNode can only provide an old state of the namespace
created during latest checkpoint.
Also CheckpointNode is a replacement for SecondaryNameNode in earlier releases.

BackupNode does checkpoints too, but in addition keeps an up-to-date state
of the namespace in its memory.
When the primary NN dies you can ask BackupNode to save namespace, which will
create the up-to-date image, and then start NN instead of BN on the node BN
was running using that saved image directly or start NN on a different node
using importCheckpoint from the saved inage directory.

See the guide here.
http://hadoop.apache.org/hdfs/docs/r0.21.0/hdfs_user_guide.html#Checkpoint+Node

Thanks,
--Konstantin

On 9/8/2010 11:36 PM, ChingShen wrote:
> Hi all,
>
>   I got the Backup node(BN) that includes all the checkpoint responsibilities,
>   and it maintains an up-to-date namespace state, which is always in
> sync with the active NN.
>
>   Q1. In which situation do we need a CN?
>
>   Q2. If the NameNode machine fails, which different manual
> intervention between BN and CN?
>
> Thanks.
>
> Shen
>