Re: [nfsv4] can a server replace a read deleg with a write deleg?

[Trond Myklebust <[e-mail]>]

On Sun, 2010-07-04 at 12:28 -0400, Rick Macklem wrote:
> Somewhat tangencial to the recent thread, what about the following
> simple case:
> - client opens foo for reading and gets read_deleg_stateid_foo
> - client opens foo for writing against the server (which it must do
>    when it holds a read delegation)
> 
> Now, if the server sees that this client is the only one with a read
> delegation for (and opens on) "foo", it could issue a write delegation
> to the client for "foo".
> - To do this does it first need to CBRecall the read delegation? (If so,
>    it probably cannot issue the write delegation for this open, since it
>    would take too long to reply to the Open unless it returns NFS4ERR_DELAY
>    for the Open for a while. Not an ideal situation.)
> OR
> - Can it return a write delegation for "foo" in the write open reply?
>    - If it is allowed to do this, does this delegation replace
>      read_delegation_stateid_foo or is there now multiple delegations for
>      "foo" issued to the same client?
>      (I don't like the concept of having multiple delegations issued to
>       the same client for the same file concurrently and I'm pretty sure
>       my client isn't implemented to handle this case. Without looking at
>       the code to be sure, I think it logs an error and throws away the
>       new second delegation.)
> 
> I don't think this is clarified in RFC3530, but please correct me if I'm
> incorrect w.r.t. this.

I read RFC3530 as being perfectly clear on this.

In section 9.4 it explicitly states that the client _cannot_ make any
changes to the contents or attributes of the file if it holds a read
delegation.

In section 9.4.4 it states that the server must recall the delegation in
the event of a conflicting OPEN or READ/WRITE w/ special stateids.

Cheers
  Trond