Re: client's caching of server-side capabilities

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

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.