Re: File system awareness (or lack thereof) of vfs granting of leases

[David Teigland <teigland@redhat.com>]

On Tue, Feb 20, 2007 at 03:51:54PM -0500, bfields@fieldses.org wrote:
> > On Tue, Feb 20, 2007 at 10:46:51AM -0500, Robert Rappaport wrote:
> > We did an experimental distributed lease implementation in gfs(1) a while
> > ago.  It worked, but was so extremely expensive that there was no point in
> > considering it seriously.  The problem is that _every_ open and close of
> > every file requires a new dlm lock operation.  Leases require knowledge
> > about the cluster-wide opened/closed state of files, not only that but the
> > mode they're open in.
> 
> We're using leases to implement NFSv4 delegations.  Delegations are
> similar to leases--they come in read and write variants, and they give
> clients a guarantee that they'll be warned before another client is
> allowed to do a conflicting open--but delegations are completely optional.
>  A server can deny a delegation for any reason, even when there isn't
> necessarily a conflicting open.
> 
> So perhaps we need some way for nfsd to ask the filesystem to give it a
> lease, but only if it's easy to do so.  Would it be possible to make it
> cheap for GFS to give out leases in some particular (hopefully common)
> cases?

I don't know of any shortcuts off hand, but there could certainly be some.
Doing something completely different and not using cluster locks may also
be worth investigating.

Dave