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

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...

Thanks!  Maybe that's why I couldn't find it.  I was looking through 
sysctl settings, which only run through /proc (I guess).  I keep 
forgetting /sys exists now.

Philip

<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

Yeah, these definitely have no effect on how much is being read from the 
disks.  There is a direct correlation, though, between the chunk size on 
the raid0 portion of the raid10 and the amount of data that is read.  If 
you increase the chunk size, you increase not just the number of bytes 
read from the disks.

There's something seriously messed up with software RAID10.  It's 
practically unusable for high-load situations where before it was 
perfectly fine.