Re: client's caching of server-side capabilities

On Wed, 2021-06-30 at 11:22 -0400, J. Bruce Fields wrote:

On Tue, Jun 29, 2021 at 01:51:43PM +0000, Chuck Lever III wrote:

quoted

quoted

On Jun 29, 2021, at 9:48 AM, Olga Kornievskaia [off-list ref]
wrote:

On Tue, Jun 29, 2021 at 8:58 AM Chuck Lever III
[off-list ref] wrote:

quoted

quoted

On Jun 28, 2021, at 6:06 PM, Trond Myklebust
[off-list ref] wrote:

On Mon, 2021-06-28 at 16:23 -0400, Olga Kornievskaia wrote:

quoted

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.

I 100% agree with the above. However...

quoted

Detection of new features, etc
can wait until the client needs to restart.

A server reboot can be part of a failover to a different
server. I
think capability discovery needs to happen as part of server
reboot
recovery, it can't be optimized away.

Can you clarify what you mean by a "failover to a different
server"?

IP-based failover means that a server can crash, and its partner
can
detect that and take over the IP address and exports of the failed
server. The replacement server doesn't have to have exactly the
same
set of capabilities.

So it could also lose capabilities?

I'm a little nervous about server features being changed out from
under
the client while the client has the server mounted.

But, I don't know, looking quickly through the list of NFS_CAP_*
definitions in nfs_fs_sb.h, I'm not coming up with a case where we
couldn't handle it, maybe it's OK.

--b.

I'm not taking any patches for the server reboot case. If someone wants
to do it for the migration case, then fine: that's not a case that is
common or that requires performance. However reprobing all mounted
filesystems on every server reboot is NACKed.

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