@Vijay : We seem to be in 100% sync though :) Warm Regards, Tariq https://mtariq.jux.com/ cloudfront.blogspot.com
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, > Tariq > https://mtariq.jux.com/ > cloudfront.blogspot.com > > > 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. >>> -or- >>> 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. >>> >>> Thanks, >>> Rahul >>> >>> >>> >> >
