Bonnie getc reading hitting exact 65536 kb/s

["A. Liemen" <maillist@xxxxxxxxxx>]

Hi,

during some xfs tuning tests I discovered the following phenomenon. 
Maybe someone can explain it to me:

Bonnie++ Reading with getc()... hits exactly an almost constant
65536 kbyte/s with only ~15% IO usage.

iostat and bonnie output can be found under:

http://alexander.liemen.net/bonnie.txt

sdb is a 14 disk RAID10 (128kb stripe) 2.8Tbyte XFS Volume with an 
external log (128Mb) on sda7 (Raid1)

mount options are:

noatime,nodiratime,logbufs=8,ihashsize=65536,logdev=/dev/sda7

mkfs:

mkfs.xfs -f -d sunit=256,swidth=1792 -l 
version=2,size=128m,logdev=/dev/sda7  /dev/sdb

scheduler: deadline

system:
- debian testing 2.6.15-1-amd64-k8-smp
- dual opteron 275-285 (both tested)
- 16gb ram
- areca pci-x 1160 controller 1gb cache v1.41 firmware
- WD400YR disks

CPU usage is 100% (on one of the cpus) but i don't think that's the 
limiting factor since it hits that 16bit value absolut constantly.

I have also tested it with exactly the same server but faster CPUs and 
it's hitting the same barrier + tested it with no mkfs.xfs options + no 
mount options on a second identical server. Same problem.

Intelligent reading is way above with 350+ Mb/s (check bonnie.txt)


Anyone got an idea why it is hitting that 16bit value / barrier?

Also of interest would be why the random seeks are so bad.


Thanks a lot
Alex