Re: Read/Write NFS I/O performance degraded by FLUSH_STABLE page flushing

[Trond Myklebust <trond.myklebust@fys.uio.no>]

Look... This happens when you _flush_ the file to stable storage if
there is only a single write < wsize. It isn't the business of the NFS
layer to decide when you flush the file; that's an application
decision...

Trond



On Fri, 2009-05-29 at 11:55 -0400, Brian R Cowan wrote:
> Been working this issue with Red hat, and didn't need to go to the list... 
> Well, now I do... You mention that "The main type of workload we're 
> targetting with this patch is the app that opens a file, writes < 4k and 
> then closes the file." Well, it appears that this issue also impacts 
> flushing pages from filesystem caches.
> 
> The reason this came up in my environment is that our product's build 
> auditing gives the the filesystem cache an interesting workout. When 
> ClearCase audits a build, the build places data in a few places, 
> including:
> 1) a build audit file that usually resides in /tmp. This build audit is 
> essentially a log of EVERY file open/read/write/delete/rename/etc. that 
> the programs called in the build script make in the clearcase "view" 
> you're building in. As a result, this file can get pretty large.
> 2) The build outputs themselves, which in this case are being written to a 
> remote storage location on a Linux or Solaris server, and
> 3) a file called .cmake.state, which is a local cache that is written to 
> after the build script completes containing what is essentially a "Bill of 
> materials" for the files created during builds in this "view."
> 
> We believe that the build audit file access is causing build output to get 
> flushed out of the filesystem cache. These flushes happen *in 4k chunks.* 
> This trips over this change since the cache pages appear to get flushed on 
> an individual basis.
> 
> One note is that if the build outputs were going to a clearcase view 
> stored on an enterprise-level NAS device, there isn't as much of an issue 
> because many of these return from the stable write request as soon as the 
> data goes into the battery-backed memory disk cache on the NAS. However, 
> it really impacts writes to general-purpose OS's that follow Sun's lead in 
> how they handle "stable" writes. The truly annoying part about this rather 
> subtle change is that the NFS client is specifically ignoring the client 
> mount options since we cannot force the "async" mount option to turn off 
> this behavior.
> 
> =================================================================
> Brian Cowan
> Advisory Software Engineer
> ClearCase Customer Advocacy Group (CAG)
> Rational Software
> IBM Software Group
> 81 Hartwell Ave
> Lexington, MA
>  
> Phone: 1.781.372.3580
> Web: http://www.ibm.com/software/rational/support/
>  
> 
> Please be sure to update your PMR using ESR at 
> http://www-306.ibm.com/software/support/probsub.html or cc all 
> correspondence to sw_support@us.ibm.com to be sure your PMR is updated in 
> case I am not available.
> 
> 
> 
> From:
> Trond Myklebust <trond.myklebust@fys.uio.no>
> To:
> Peter Staubach <staubach@redhat.com>
> Cc:
> Chuck Lever <chuck.lever@oracle.com>, Brian R Cowan/Cupertino/IBM@IBMUS, 
> linux-nfs@vger.kernel.org
> Date:
> 04/30/2009 05:23 PM
> Subject:
> Re: Read/Write NFS I/O performance degraded by FLUSH_STABLE page flushing
> Sent by:
> linux-nfs-owner@vger.kernel.org
> 
> 
> 
> On Thu, 2009-04-30 at 16:41 -0400, Peter Staubach wrote:
> > Chuck Lever wrote:
> > >
> > > On Apr 30, 2009, at 4:12 PM, Brian R Cowan wrote:
> > >>
> > >> 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=ab0a3dbedc51037f3d2e22ef67717a987b3d15e2
> 
> > >>
> > Actually, the "stable" part can be a killer.  It depends upon
> > why and when nfs_flush_inode() is invoked.
> > 
> > I did quite a bit of work on this aspect of RHEL-5 and discovered
> > that this particular code was leading to some serious slowdowns.
> > The server would end up doing a very slow FILE_SYNC write when
> > all that was really required was an UNSTABLE write at the time.
> > 
> > Did anyone actually measure this optimization and if so, what
> > were the numbers?
> 
> As usual, the optimisation is workload dependent. The main type of
> workload we're targetting with this patch is the app that opens a file,
> writes < 4k and then closes the file. For that case, it's a no-brainer
> that you don't need to split a single stable write into an unstable + a
> commit.
> 
> So if the application isn't doing the above type of short write followed
> by close, then exactly what is causing a flush to disk in the first
> place? Ordinarily, the client will try to cache writes until the cows
> come home (or until the VM tells it to reclaim memory - whichever comes
> first)...
> 
> Cheers
>   Trond
> 
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