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Message-Id: <81BBEA25-CDE5-42B1-996D-90C26A43DEF6@oracle.com> Date: Thu, 25 Apr 2013 14:40:11 -0400 From: Chuck Lever <chuck.lever@...cle.com> To: "bfields@...ldses.org" <bfields@...ldses.org> Cc: "Myklebust, Trond" <Trond.Myklebust@...app.com>, David Wysochanski <dwysocha@...hat.com>, Dave Chiluk <chiluk@...onical.com>, "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY On Apr 25, 2013, at 2:19 PM, "bfields@...ldses.org" <bfields@...ldses.org> wrote: > On Thu, Apr 25, 2013 at 02:10:36PM +0000, Myklebust, Trond wrote: >> On Thu, 2013-04-25 at 09:49 -0400, bfields@...ldses.org wrote: >>> On Thu, Apr 25, 2013 at 01:30:58PM +0000, Myklebust, Trond wrote: >>>> On Thu, 2013-04-25 at 09:29 -0400, bfields@...ldses.org wrote: >>>> >>>>> My position is that we simply have no idea what order of magnitude even >>>>> delay should be. And that in such a situation exponential backoff such >>>>> as implemented in the synchronous case seems the reasonable default as >>>>> it guarantees at worst doubling the delay while still bounding the >>>>> long-term average frequency of retries. >>>> >>>> So we start with a 15 second delay, and then go to 60 seconds? >>> >>> I agree that a server should normally be doing the wait on its own if >>> the wait would be on the order of an rpc round trip. >>> >>> So I'd be inclined to start with a delay that was an order of magnitude >>> or two more than a round trip. >>> >>> And I'd expect NFS isn't common on networks with 1-second latencies. >>> >>> So the 1/10 second we're using in the synchronous case sounds closer to >>> the right ballpark to me. >> >> OK, then. Now all I need is actual motivation for changing the existing >> code other than handwaving arguments about "polling is better than flat >> waits". >> What actual use cases are impacting us now, other than the AIX design >> decision to force CLOSE to retry at least once before succeeding? > > Nah, I've got nothing, and I agree that the AIX problem is there bug. > > Just for fun I looked at re-checked the Linux server cases. As far as I > can tell they are: > > - delegations: returned immediately on detection of any > conflict. The current behavior in the sync case looks > reasonable to me. > - allocation failures: not really sure it's the best error, but > it seems to be all the protocol offers. We probably don't > care much what the client does in this case. > - some rare cases that would probably indicate bugs (e.g., > attempting to destroy a client while other rpc's from that > client are running.) Again we don't care what the client does > here. > - the 4.1 slot-inuse case. > > We also by default map four errors (ETIMEDOUT, EAGAIN, EWOULDBLOCK, > ENOMEM) to delay. I thought I remembered one of those being used by > some HFS system, but can't actually find an example now. A quick grep > doesn't show anything interesting. It's worth mentioning that servers that have frozen state (say, in preparation for Transparent State Migration) may use NFS4ERR_DELAY to prevent clients from modifying open or lock state until that state has transitioned to a destination server. -- Chuck Lever chuck[dot]lever[at]oracle[dot]com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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