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.
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