[Federated-fs] Federated FS, pNFS, NFSv4.1 referrals....

[Daniel Ellard]

Christian Bandulet - Principal Engineer wrote:
> Daniel,
>
> I had a look at:
>
> http://tools.ietf.org/id/draft-ellard-nfsv4-federated-fs-01.txt
>
> I am still not sure what the difference of federated FS is compared to 
> pNFS or NFSv4.1. referrals.
Good question.

fed-fs, pNFS, and NFSv4.1 address different issues, and so instead of 
focusing on their differences, I think it's more informative to focus on 
how they complement each other.  They are not competing ideas -- they 
augment each other.
> I guess that pNFS and NFSv4.1. referrals should also work across 
> heterogeneous file server, right?
Yes!  If they don't then something is broken.  The semantic and 
structure of these referrals are part of the protocols and should be 
platform-independent.  If not, then fed-fs is broken as well.
> Is it correct that a NFSv4.1. client which requests a file from server 
> A will get back a referral structure instead of the file itself. the 
> client then in a second step has to contact the right file server 
> whose name will be part of the referral structure received from server A?
>
One of the key differences between pNFS and fed-fs is that the client is 
"aware" of pNFS (the client knows, from info it finds from the server, 
how to piece together files from shares spread across a set of servers), 
while the clients don't need to know whether fed-fs is being used 
(although they might suspect something is up if they keep getting 
referrals...).  fed-fs is the protocol that servers use to figure out 
where to refer clients, while pNFS includes a client protocol.
> In a federated FS would  server A be a kind of proxy -  i.e. the 
> client will receive the requested file  from server A no matter 
> whether it is really physically hosting the file? i.e. server A can 
> get the file from server B and deliver it to the client?
No -- fed-fs does not proxy requests.  If server A has a junction to 
directory X and directory X happens to be hosted on server B, then what 
would happen if a client of A accesses directory X is that the client 
would get a referral to X on B.
> Is federated FS e.g. about unifying several pNFS-built global 
> namespaces to a single super-global namespace?
That's one use case.   It can be used to stitch together any kind of 
NFSv4 namespaces together.  fed-fs doesn't care whether the underlying 
filesystems use pNFS or vanilla NFSv4.  It only tells the servers what 
they need to know in order to refer the clients to the right places -- 
after that, it's up to the client to access the objects via pNFS or any 
other way.

-Dan