troubleshooting I/O performance bottleneck

[manu%netbsd.org@localhost (Emmanuel Dreyfus)]

Hello

I run into a disk I/O bottleneck problem on a i386 xen setup:
The problem is exhibited by performuing writes:
# time dd if=/dev/zero of=test bs=1024k count=100

On the dom0 (xen 3.1.4, netbsd-4 as 20070707), it is fair
    2.05s real     0.00s user     0.57s system

On any domU (netbsd-4 or netbsd-5), it is weak:
    5.94s real     0.00s user     0.73s system

running systat vm on the dom0 during the I/O from domU gives the ouput above

 Any idea of where I should look at?

    1 user     Load  0.24  0.06  0.02                  Sat Apr 25 07:08:55

Proc:r  d  s  w     Csw    Trp    Sys   Int   Sof    Flt   PAGING SWAPPING
           7       2182    185    410  1221   297      9   in  out in  out
                                                        ops
   2.0% Sy   0.0% Us   0.0% Ni   7.0% In  91.0% Id    pages
|    |    |    |    |    |    |    |    |    |    |
=%%%%                                                                forks
                                                                     fkppw
           memory totals (in kB)            1045 Interrupts          fksvm
          real  virtual     free             100 vcpu0 softclock     pwait
Active   25812    25812   161000               5 vcpu0 softnet       relck
All      86388    86388  8160992              79 vcpu0 xenevt        rlkok
                                                 vcpu0 xencons       noram
Namei         Sys-cache     Proc-cache         6 ioapic1 pin 0       ndcpy
    Calls     hits    %     hits     %           ioapic0 pin 16      fltcp
        6        6  100                      738 ioapic0 pin 19      zfod
                                                 vcpu0 ioapic0       cow
Disks:   md0   cd0   wd0   wd1 raid0         117 vcpu0 clock      64 fmin
 seeks                                           vcpu0 xenbus     85 ftarg
 xfers               366   363   366             vcpu0 xbd3.3        itarg
 bytes               11M   11M   11M             vcpu0 xvif3.0  3863 wired
 %busy              96.0  39.0   100             vcpu0 xbd13.3       pdfre
                                                 vcpu0 xvif13.0
-- 
Emmanuel Dreyfus
http://hcpnet.free.fr/pubz
manu%netbsd.org@localhost