From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philip Molter Subject: Re: Software RAID & Filesystem Cache Date: Thu, 05 Aug 2004 12:34:00 -0500 Sender: linux-raid-owner@vger.kernel.org Message-ID: <41126F88.7040006@corp.texas.net> References: <411260FB.4080604@corp.texas.net> <411268C7.9070804@h3c.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <411268C7.9070804@h3c.com> To: Mike Hardy Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Mike Hardy wrote: > > I was just researching a similar problem. I turned up a few things that > pointed at the filesystem readahead values/strategies, apparently those > things changed quite a bit between 2.4 and 2.6. You might try twiddling > those knobs, and you can find some linux-kernel threads where various > values were tested > > Also, for my specific case, I found that the default I/O scheduler in > Fedora Core 2, the "Complete Fair Queueing" scheduler ('cfq') wasn't as > good as the 'deadline' scheduler, and neither was good as the > 'anticipatory' scheduler. So now I boot with the kernel command line > parameter 'scheduler=as' and things are faster. That's extremely > workload specific though - so you might run your own scheduler derby and > see what works. > > The readahead is the first place I'd look though. All in all, it appears > that 2.6 kernels need a great deal more I/O tuning before they can be > put in production. While I like the flexibility that's available, the > default settings seem to be a major negative change from 2.4. This sort > of thing is just now being quantified and hopefully it gets sorted out > in the next couple of releases Hi Mike, Thanks for the response. I run the system under the anticipatory scheduler, too, but I tried all of this under the default, deadline and as schedulers with no difference between the two. My first suspicion was readahead as well, but I couldn't find any of the sysctl options to control readahead in the kernel (as there were under 2.4). I did mess around with readahead directly on the drives, but as expected, that had no effect. I tried other reading options, but none seemed to have any effect. The RAID seemed very insistent on reading all of that data. I was flummoxed. Philip