[linux-lvm] Why can't I get more bandwidth by adding more disks?

[Xiaoxiang Liu]

I have tried to upgrade LVM to beta8 on kernel 2.4.7 and it
works well. Then I am trying to do some raw io test to see
how much bandwidth I can get. My machine is HP kayak and connected
with a MTI RAID by fiber channel.

If I do sequence rawio on one scsi disk in MTI, the bandwidth is 14MB/s.
Then I create a LV on two disks with striping size 4k, the bandwidth
becomes 28MB/s and I really feel excited for this result. :)
But when I create LV on more disks (3,4,5 or 8 disks), the bandwidth
can't increase any more. I always get 28MB/s or less. I do 1G-byte sequence 
rawio and my buffer size is 32k. The maximum bandwidth of fiber channel is
100MB/s and we could actually get 75MB/s in other tests.
 
I don't know what's the bottleneck here? If I create LV on 4 disks with
striping size 4K and do 32k rawio, does and how LVM do parallel io on
those 4 disks? Why can't I increase the bandwidth by 4 times? 
Please help me! Your suggestions will be highly appreciated!


Xiang