[Pkg-xen-devel] Slow IO on DomUs under Xen 4.1 kernel 3.2.0.4

[Iñaki Martinez de Ilarduya]

Hello,

I hope this is the mailing list to discuss this issue, but if not sorry 
for spamming!

I just discovered a really slow IO performance on DomUs running debian 
wheezy over wheezy dom0s, any operation that requires disk read or write 
takes a lot of time.

Here are the hardware parameters:

Single Xeon 5120 (dual core)
8GB of ram
Hardware raid 2x250GB drives raid 1

his is the configuration of the domU:


# Configuration file for the Xen instance cri4-git, created
# by xen-tools 4.3.1 on Fri Jan 10 11:01:24 2014.

#  Kernel + memory size
#
bootloader = '/usr/lib/xen-4.1/bin/pygrub'

vcpus       = '1'
memory      = '2048'

#  Disk device(s).
root        = '/dev/xvda2 ro'
disk        = [
                   'phy:/dev/vg0/cri4-git-root,xvda2,w',
                   'phy:/dev/vg0/cri4-git-home,xvda3,w',
                   'phy:/dev/vg0/cri4-git-swap,xvda1,w',
               ]
#
#  Hostname
#
name        = 'cri4-git'

#  Networking

vif         = [ 'ip=10.5.62.7 ,mac=00:16:3E:ED:82:FC,bridge=xenbr0' ]

on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'


I have reserved 1 core and 1GB of ram to the Dom0, after reading that it 
should improve general performance. However, I get similar results with 
and without pinning.



Here are some tests I have run on the domU

  time dd if=/dev/zero of=/home/testzero bs=8k count=16000; sync
  16000+0 records in
  16000+0 records out
  131072000 bytes (131 MB) copied, 27.6045 s, 4.7 MB/s

  real    0m27.644s
  user    0m0.008s
  sys     0m0.368s

During the process, the iowait on the domU is around 50% and on the dom0 0%.
The fstab of the domU:

# /etc/fstab: static file system information.
#
# <file system> <mount point>   <type> <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0
devpts          /dev/pts        devpts rw,noexec,nosuid,gid=5,mode=620 0  0
/dev/xvda1 none swap sw 0 0
/dev/xvda2 / ext4 sync,errors=remount-ro 0 1
/dev/xvda3 /home ext4 nodev,nosuid 0 1

If I try the same dd command, but writing to xvda2, the speed is even lower.

Other information:
Tried another domU, this time with squeeze, but results are the same.

I have another machine, with different hardware specs, but same software 
configuration and results are the same.

This issue does not happen with debian squeeze as Dom0 running 
2.6.32-5-xen-amd64.

I have been googling around but so far found nothing related to this 
issue...

Regards


-- 
Iñaki Mtz de Ilarduya