Return-Path: linux-nfs-owner@vger.kernel.org Received: from fieldses.org ([174.143.236.118]:45467 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759163Ab3DYSTf (ORCPT ); Thu, 25 Apr 2013 14:19:35 -0400 Date: Thu, 25 Apr 2013 14:19:32 -0400 From: "bfields@fieldses.org" To: "Myklebust, Trond" Cc: David Wysochanski , Dave Chiluk , "linux-nfs@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY Message-ID: <20130425181932.GA5049@fieldses.org> References: <1366836949-18465-1-git-send-email-chiluk@canonical.com> <1366838926.22397.25.camel@leira.trondhjem.org> <5178549A.7010402@canonical.com> <1366842905.22397.49.camel@leira.trondhjem.org> <1366892374.26249.294.camel@localhost.localdomain> <20130425132907.GB31851@fieldses.org> <1366896654.4719.18.camel@leira.trondhjem.org> <20130425134918.GC31851@fieldses.org> <1366899034.6812.4.camel@leira.trondhjem.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1366899034.6812.4.camel@leira.trondhjem.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Thu, Apr 25, 2013 at 02:10:36PM +0000, Myklebust, Trond wrote: > On Thu, 2013-04-25 at 09:49 -0400, bfields@fieldses.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@fieldses.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. --b.