Re: Switch vfs.nfsd.issue_delegations to TUNABLE ?

On Tue, 28 Nov 2017 14:12:38 +0000
Rick Macklem <rmacklem_at_uoguelph.ca> wrote:

> Konstantin Belousov wrote:
> >On Tue, Nov 28, 2017 at 02:26:10PM +0100, Emmanuel Vadot wrote:
> >> On Tue, 28 Nov 2017 13:04:28 +0200
> >> Konstantin Belousov <kostikbel_at_gmail.com> wrote:
> >>
> >> > On Tue, Nov 28, 2017 at 11:41:36AM +0100, Emmanuel Vadot wrote:
> >> > >
> >> > >  Hello,
> >> > >
> >> > >  I would like to switch the vfs.nfsd.issue_delegations sysctl to a
> >> > > tunable.
> Since it defaults to "disabled", I don't see why a tunable would be necessary?
> (Just do nothing and delegations don't happen. If you want the server
>  to issue delegations, then use the sysctl to turn them on. If you want to turn
>  them off again at the server, reboot the server without setting the sysctl.)

 If you need to reboot to make things working again without delegation
this shouldn't be a sysctl.

> > > >  The reason behind it is recent problem at work on some on our filer
> > > > related to NFS.
> > > >  We use NFSv4 without delegation as we never been able to have good
> > > > performance with FreeBSD server and Linux client (we need to do test
> > > > again but that's for later).
> Delegations are almost never useful, especially with Linux clients.

 Can you elaborate ? Reading what delegation are I see that this is
exactly what I'm looking for to have better performance with NFS for my
workload (I only have one client per mount point).

> [stuff snipped]
> > > Perhaps make nodeleg per-mount flag ?
> The sysctl vfs.nfsd.issue_delegations only affects the server.
> If you have a FreeBSD client that is mounting a delegations enabled server and
> does not want delegations, just don't run the "nfscbd" daemon on the client.
> The only time you want the "nfscbd" daemon running is for delegations and
> pNFS layouts. (I suppose there is the case of a client using NFSv4.1/pNFS against
> a server with delegations enabled, but since delegations aren't very useful,
> I'd just disable delegations on the server for this case.)
> 
> Having a per-mount version of this would be overkill, I think. It would have to
> disable callbacks on the mount point, since there is no way for a client to say
> "don't give me delegations" except disabling callbacks, which the server
> requires for delegation issue.
> [stuff snipped]
> The case where there has never been delegations issued will result in an
> empty delegation queue. Maybe this case can be handled without acquiring
> the "global NFSv4 state lock", which is what sounds like is causing the
> performance issue. Maybe a global counter of how many delegations are
> issued that is handled by atomic ops.
> --> If it is 0, nfsrv_checkgetattr() can just return without acquiring the lock.
> 
> I'll take a look at this, rick
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-- 
Emmanuel Vadot <manu_at_bidouilliste.com> <manu_at_freebsd.org>