Re: client's caching of server-side capabilities

From: Olga Kornievskaia <aglo@umich.edu>
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: client's caching of server-side capabilities
Date: Tue, 29 Jun 2021 09:41:49 -0400	[thread overview]
Message-ID: <CAN-5tyHTavHRmQKGhUmBvngMwAAK2pr89zBuppK_s27HduJfqQ@mail.gmail.com> (raw)
In-Reply-To: <81154dc28d528402bf5e090a81e6892c7abc431c.camel@hammerspace.com>

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.

Thank you for the feedback.

>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>