KVM and Hylafax

[Federico Fanton <federico@panizzolo.it>]

Hi everyone!
We have an Hylafax 5.2.5 CentOS 5 installation hosted inside a Xen 
virtual machine. It works quite well, but now I'm in the process of 
upgrading/migrating it to a KVM virtual machine running Ubuntu 10.04.

The problem I'm having is that - while receiving works fine - sending 
faxes is extremely unreliable, I get lots of "No response to MPS 
repeated 3 tries", or "Failure to transmit clean ECM image data", just 
what would happen if the phone line was noisy.

Since the line, modem and configuration files I'm using are the same as 
before, I thought that it could be a KVM scheduling issue, but even 
setting cpu_shares to 10240 instead of 1024 doesn't change a thing...

I've tried asking on the Hylafax ML, and they say that

"generally speaking I'm very skeptical of real-time performance for 
virtualized machines and especially virtualized serial hardware. 
Because fax is time-sensitive it's certainly possible for the virtual 
machine to not be attentive-enough to it."

Are there any success stories on running Hylafax on KVM? What could I 
try to make it more "attentive"?


Here's how I'm mapping the serial port (where the modem is connected):

<serial type='dev'>
    <source path='/dev/ttyS0'/>
    <target port='0'/>
</serial>


And how I tuned the VM:

<cputune>
   <shares>10240</shares>
   <period>100000</period>
   <quota>-1</quota>
</cputune>


Many thanks for your time!