Message-ID: <20130425132907.GB31851@fieldses.org>
Date:	Thu, 25 Apr 2013 09:29:07 -0400
From:	"bfields@...ldses.org" <bfields@...ldses.org>
To:	David Wysochanski <dwysocha@...hat.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.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 Thu, Apr 25, 2013 at 08:19:34AM -0400, David Wysochanski wrote:
> On Wed, 2013-04-24 at 22:35 +0000, Myklebust, Trond wrote:
> > On Wed, 2013-04-24 at 16:54 -0500, Dave Chiluk wrote:
> > > On 04/24/2013 04:28 PM, Myklebust, Trond wrote:
> > > > On Wed, 2013-04-24 at 15:55 -0500, Dave Chiluk wrote:
> > > >> Changing the retry to start at NFS4_POLL_RETRY_MIN and exponentially grow
> > > >> to NFS4_POLL_RETRY_MAX allow for faster handling of these error conditions.
> > > >>
> > > >> Additionally this alleviates an interoperability problem with the AIX NFSv4
> > > >> Server.  The AIX server frequently (2 out of 3) returns NFS4ERR_DELAY, on a
> > > >> close when it happens in close proximity to a RELEASE_LOCKOWNER.  This would
> > > >> cause a linux client to hang for 15 seconds.
> > > > 
> > > > Hi Dave,
> > > > 
> > > > The AIX server is not being motivated by any requirements in the NFSv4
> > > > spec here, so I fail to see the reason why the behaviour that you
> > > > describe can justify changing the client. It is not at all obvious to me
> > > > that we should be retrying aggressively when NFSv4 servers return
> > > > NFS4ERR_DELAY. What makes 1/10sec more correct in these situations than
> > > > the exising 15 seconds?
> > > 
> > > I agree with you that AIX is at fault, and that the preferable situation
> > > for the linux client would be for AIX to not return NFS4ERR_DELAY in
> > > this use case.  I have attached a simple program that causes exacerbates
> > > the problem on the AIX server.  I have already had a conference call
> > > with AIX NFS development about this issue, where I vehemently tried to
> > > convince them to fix their server.  Unfortunately as I don't have much
> > > reputation in the NFS community, I was unable to convince them to do the
> > > right thing.  I would be more than happy to set up another call, if
> > > someone higher up in the linux NFS hierarchy would be willing to
> > > participate.
> > 
> > I'd think that if they have customers that want to use Linux clients,
> > then those customers are likely to have more influence. This is entirely
> > a consequence of _their_ design decisions, quite frankly, since
> > returning NFS4ERR_DELAY in the above situation is downright silly. The
> > server designers _know_ that the RELEASE_LOCKOWNER will finish whatever
> > it is doing fairly quickly; it's not as if the CLOSE wouldn't have to do
> > the exact same state manipulations anyway...
> > 
> > > That being said, I think implementing an exponential backoff is an
> > > improvement in the client regardless of what AIX is doing.  If a server
> > > needs only 2 seconds to process a request for which NFS4ERR_DELAY was
> > > returned, this algorithm would get the client back and running after
> > > only 2.1 seconds of elapsed time.  Whereas the current dumb algorithm
> > > would simply wait 15 seconds.  This is the reason that I implemented
> > > this change.
> > 
> > Right, but my point above is that _in_general_ if we don't know why the
> > server is returning NFS4ERR_DELAY, then how can we attach any retry
> > numbers at all? HSM systems, for instance, have very different latencies
> > than the above and were the reason for inventing NFS3ERR_JUKEBOX in the
> > first place.
> > 
> 
> Agreed we can't know why the server is returning NFS4ERR_DELAY so it's
> hard to pick a retry number.  Can you explain the rationale for the
> current 15 seconds delay?  Was it just for simplicity or something else?

As I understand it the original idea was that cold data really could
take multiple seconds or minutes to retrieve (because e.g. a tape
library might need to go load the right tape and rewind to the right
spot...).  Is that sort of system really used much these days?

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.

--b.
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