Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY
From: Myklebust, Trond
Date: Wed Apr 24 2013 - 18:35:17 EST
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
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
> > The motivation for doing it in the case of OPEN, SETATTR, etc is
> > clearer: those operations may require the server to recall a delegation,
> > in which case aggressive retries are in order since delegation recalls
> > are usually fast.
> > The motivation in the case of LOCK is less clear, but it is basically
> > down to the fact that NFSv4 has a polling model for doing blocking
> > locks.
> > In all other cases, why should we be treating NFS4ERR_DELAY any
> > differently from how we treat NFS3ERR_JUKEBOX in NFSv3?
> > Note that if we do decide that changing the client is the right thing,
> > then I don't want the patch to add new fields to struct rpc_task. That's
> > the wrong layer for storing NFSv4 client specific data.
> This is something that I was concerned about as well, but I could not
> find another persistent way to do this. I am open to suggestions of
> which structures would be more acceptable.
We could change nfs4_async_handle_error() to take a struct
nfs4_exception, just like nfs4_handle_exception() does; at some point we
can use that to unify the two.
Just store the timeout somewhere in the nfs4_closedata.
Linux NFS client maintainer
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