Re: Software RAID & Filesystem Cache

[Philip Molter <philip@corp.texas.net>]

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