From: Trond Myklebust <trond.myklebust@fys.uio.no>
Subject: Re: NFS file locking for clustered filesystems
Date: Wed, 11 Aug 2004 22:34:45 -0700
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Cc: Olaf Kirch <okir@suse.de>, nfsv4@linux-nfs.org, nfs@lists.sourceforge.net
To: Sridhar Samudrala <sri@us.ibm.com>
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Errors-To: nfs-admin@lists.sourceforge.net

P=E5 on , 11/08/2004 klokka 14:35, skreiv Sridhar Samudrala:

> So, in the case of a client which doesn't allow blocking, are you suggest=
ing
> that the cluster filesystem to return LCK_BLOCKED immediately and start t=
he
> asynchrous call and return LCK_DENIED in a few seconds if it cannot compl=
ete
> the operation?

Sort of.

Firstly, having the filesystem return NLM-specific errors is a no-no.
Lets keep the errors generic (-EAGAIN =3D=3D -EWOULDBLOCK for instance.)
NFSv4 does not understand LCK_BLOCKED.

Secondly, I suggest that lockd should manage its timeouts itself rather
than relying on the clustered filesystem to do so. Timers are cheap...

> But i am not sure if we can handle this easily with the current design of
> lockd. As soon as the lock operation returns LCK_BLOCKED, i think svc_sen=
d() is
> called from svc_process() which will send a reply with NLM_BLOCKED.

For NFSv4 this is not a problem: NFS4ERR_DELAY will make the client back
off and retry the request later.

For NLM, you are clearly going to have to figure out a way around the
above problem. (but that's why you get paid the big bucks right? ;-))
Note that the sunrpc server code already has a mechanism for deferring
requests when dealing with blocking behaviour (in case it needs to do an
upcall) and then replaying them later. Perhaps you could look into
reusing that here?

Cheers,
  Trond


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