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@xxxxxxxxxx to be sure your PMR is updated in case I am not available. From: Trond Myklebust <trond.myklebust@xxxxxxxxxx> To: Peter Staubach <staubach@xxxxxxxxxx> Cc: Chuck Lever <chuck.lever@xxxxxxxxxx>, Brian R Cowan/Cupertino/IBM@IBMUS, linux-nfs@xxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxx 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 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
