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Re: Questions about BN and CN
Hi Shen,

Why do we need CheckpointNode?
1. First of all it is a compatible replacement of SecondaryNameNode.
2. Checkpointing is also needed for periodically compacting edits.
You can do it with CN or BN, but CN is more lightweight.
I assume there could be cases when streaming edits to BN over network
can be slower than writing them to disk, so you might want to turn BN
off for performance reasons.
3. Also in current implementation NN allows only one BN, but multiple CNs.
So if the single BN dies the checkpointing will stall.
You can prevent it by starting two CNs instead, or one BN and one CN.
But I agree with you CN is just a subset of BN by its functionality.


On 9/22/2010 5:50 PM, ChingShen wrote:
> Thanks Konstantin,
>    But, my main question is that because the CN can only provide an old state
> of the namespace, so why do we need it? I think the BN is best solution.
> Shen
> On Thu, Sep 23, 2010 at 5:20 AM, Konstantin Shvachko<[EMAIL PROTECTED]>wrote:
>> 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