Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY

["Myklebust, Trond" <Trond.Myklebust@netapp.com>]

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.

> > 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.


-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com