Re: Software RAID & Filesystem Cache

Philip Molter wrote:

Mike Hardy wrote:
       [ ... snip ... ]

quoted

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

[ .. snip ... ]

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 

I'm honestly not trying to be a smart-ass by posting this since I
haven't played with these *a single bit* - however, this was what I found:

<berlin>/sys % find . | grep ahead
./block/hdf/queue/read_ahead_kb
./block/hdg/queue/read_ahead_kb
./block/hdc/queue/read_ahead_kb
./block/hda/queue/read_ahead_kb

Maybe those are the appropriate knobs to twiddle? If so, it appears
they've gone per-device, and they've moved to sysfs...

-Mike