-Re: NameNode failure and recovery!
Mohammad Tariq 2013-04-03, 14:58
@Vijay : We seem to be in 100% sync though :)
On Wed, Apr 3, 2013 at 8:27 PM, Mohammad Tariq <[EMAIL PROTECTED]> wrote:
> Hello Rahul,
> It's always better to have both 1 and 2 together. One common
> misconception is that SNN is a backup of the NN, which is wrong. SNN is a
> helper node to the NN. In case of any failure SNN is not gonna take up the
> NN spot.
> Yes, we can't guarantee that the SNN fsimage replica will always be up to
> date. And when you are writing the metadata on a filer or NFS, you are just
> creating an additional copy of the metadata. Don't mistake it with SNN.
> When you specify value of your "dfs.name.dir" property as a comma separated
> list, which is localFS+NFS, you are just making sure that even if something
> goes wrong with the localFS, your metadata is still same in the NFS.
> But, it is still better to have the SNN in a separate machine. But you can
> never rely 100% on SNN, because of the fact you have already mentioned.
> It'll not be in 100% sync.
> Warm Regards,
> On Wed, Apr 3, 2013 at 8:12 PM, Rahul Bhattacharjee <
> [EMAIL PROTECTED]> wrote:
>> Or both the options are used together. NFS + SNN ?
>> On Wed, Apr 3, 2013 at 8:10 PM, Rahul Bhattacharjee <
>> [EMAIL PROTECTED]> wrote:
>>> 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.
>>> 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.