Re: client's caching of server-side capabilities

[Olga Kornievskaia <aglo@umich.edu>]

On Tue, Jun 29, 2021 at 9:41 AM Olga Kornievskaia <aglo@umich.edu> wrote:
>
> On Mon, Jun 28, 2021 at 6:06 PM Trond Myklebust <trondmy@hammerspace.com> wrote:
> >
> > On Mon, 2021-06-28 at 16:23 -0400, Olga Kornievskaia wrote:
> > > Hi folks,
> > >
> > > I have a general question of why the client doesn't throw away the
> > > cached server's capabilities on server reboot. Say a client mounted a
> > > server when the server didn't support security_labels, then the
> > > server
> > > was rebooted and support was enabled. Client re-establishes its
> > > clientid/session, recovers state, but assumes all the old
> > > capabilities
> > > apply. A remount is required to clear old/find new capabilities. The
> > > opposite is true that a capability could be removed (but I'm assuming
> > > that's a less practical example).
> > >
> > > I'm curious what are the problems of clearing server capabilities and
> > > rediscovering them on reboot? Is it because a local filesystem could
> > > never have its attributes changed and thus a network file system
> > > can't
> > > either?
> > >
> > > Thank you.
> >
> > In my opinion, the client should aim for the absolute minimum overhead
> > on a server reboot. The goal should be to recover state and get I/O
> > started again as quickly as possible. Detection of new features, etc
> > can wait until the client needs to restart.
>
> Do I interpret this correctly: no capability discoveries before
> RECLAIM_COMPLETE but perhaps after? I agree that reboot recovery
> should be done as quickly as possible. If it's some time after, then
> perhaps it can be done on-demand thru say nfs sysfs api: have ability
> to clear current capabilities (or a specific one) and do discover new
> ones?
>
> The use case I'm going for is when a server upgrades and comes up with
> support for new features. Currently, it requires a client re-mount.
> But perhaps requiring "mount -o remount" in that case isn't any
> different than requiring use of sysfs.

Actually, I tried to do a "mount -o remount" after taking down the
server and changing its features (ie security label support), and the
client does not query for supported attributes. So I think either at
least perhaps that can be changed somehow or we do need a sysfs api to
be able to change server's capabilities of a given mount.

>
> Thank you for the feedback.
>
> >
> > --
> > Trond Myklebust
> > Linux NFS client maintainer, Hammerspace
> > trond.myklebust@hammerspace.com
> >
> >