Re: Software RAID & Filesystem Cache

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