On Fri, Sep 12, 2014 at 12:07 PM, Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> wrote: > On Fri, 12 Sep 2014 11:54:17 -0400 > Trond Myklebust <trondmy@xxxxxxxxx> wrote: > >> On Fri, Sep 12, 2014 at 11:21 AM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: >> > On Fri, Sep 12, 2014 at 10:36:21AM -0400, J. Bruce Fields wrote: >> >> On Fri, Sep 12, 2014 at 10:21:53AM -0400, Jeff Layton wrote: >> >> > Grace period >> >> > eventually ends, and its record is purged from the DB. >> >> > >> >> > Now we have a client that has reclaimed some files but that has no >> >> > record on stable storage. >> >> > >> >> > One possibility is to prematurely expire v4.1+ clients that have not >> >> > sent a RECLAIM_COMPLETE when the grace period ends. >> >> > >> >> > That seems problematic though -- what about clients that just happen to >> >> > do an EXCHANGE_ID just before the grace period is going to end, and >> >> > that get expired before they can issue their RECLAIM_COMPLETE. Will >> >> > that be a problem for them? >> >> >> >> In that case a client will send a reclaim, get back a NO_GRACE error, >> >> mark the rest of its state as unrecoverable, send the RECLAIM_COMPLETE, >> >> and continue normally. (To the extent it can--signalling affected >> >> processes or EIOing further attempts to use the unreclaimed state, or >> >> whatever.) >> > >> > The one thing the server *could* do in this sort of case is extend the >> > grace period by a little--I seem to recall the spec giving some leeway >> > for this kind of thing. >> >> >> Section 8.4.2.1. >> >> > So for example the server could have a heuristics like: extend the grace >> > period by another second each time we notice there's been an EXCHANGE_ID >> > or reclaim in the previous second, up to some maximum. And I suppose it >> > could also delay the grace period until someone actually attempts a >> > non-reclaim open. >> > >> > In isolation a single client slipping in the end like that sounds like a >> > freak event, but if there's a ton of state to reclaim perhaps it could >> > become more likely. >> > >> > I don't think that's a priority, we might just want to make sure we know >> > how to do that in the future. >> > >> > But now that I think about it I don't see the existing or proposed >> > nfsdcltrack stuff tying our hands in any way here. It just gives the >> > kernel some extra information, and the kernel still has discretion about >> > when exactly it wants to end the grace period. >> > >> >> It is even allowed to grant reclaim lock attempts after the grace >> period has ended _if_ and only if it can guarantee that no conflicting >> locks were issued. >> >> However note that the NFSv4.1 client is not actually allowed to issue >> non-reclaim lock requests before it has issued a RECLAIM_COMPLETE. I >> dunno how religiously we stick to that in Linux (I think we do), but >> the point is that the server can and should rely on the client >> _always_ sending a RECLAIM_COMPLETE if it is going to establish new >> locks. > > Yeah, I'm pretty sure that bit is enforced. The problem situation that > I think Bruce was referring to is this: > > Server reboots. Client1 reclaims some of its locks (but not all) and > never sends a RECLAIM_COMPLETE. Grace period ends and then server > hands out a lock to client2 that was previously held by client1 but > that didn't get reclaimed. > > Server reboots again, prior to the client1 expiring (so its record is > still in the DB). Now client1 comes back and starts reclaiming again. > This time it reclaims all of its locks and we have a conflict between > it and client2. > > It's a solvable problem, but I'll need to work through how best to do > so. > > -- That's the first edge condition described in section 8.4.3. -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
