[mythtv-users] Virtual Machines, USB tuners and PCI passthrough

Simon Hobson linux at thehobsons.co.uk
Tue Jul 20 22:20:25 UTC 2010

Tyler T wrote:

>I tried to do PCI passthough in Xen a couple years ago and remember
>lots of pain, perhaps it's better now ...

I'm running a backend as a VM with PCI passthrough. I don't seem to 
remember having had any problems with that - and it's not exactly 
what you'd call up to date versions either. I actually do passthrough 
for two cards - one tuner for the Myth backend, and a network card 
for my firewall/gateway.

>One more thought: I believe SiliconDust sells a DVB version of the HD
>HomeRun. If you go that route, a Gig-E NIC in your server should be
>able to handle 6+ tuners no sweat, and no mucking about with USB

However, you may hit another issue with Xen - network performance. 
AIUI the networking in Xen is single threaded, and so if your guests 
are hard on the network then performance can suffer. For that reason, 
using iSCSI in a guest sucks performance wise - but it's OK if you do 
the iSCSI in Dom0 and pass the result block device through as a 
virtual block device to the DomU.

As a reference point, I run a single AMD64-2000 processor, 4G ram, a 
single 1.5TB WD Green drive, and a number of guests (mostly fairly 
lightly loaded). I've a single HVR 1110 PCI DVB tuner setup with 3 
virtual tuners in MythTV. It will happily record two programs while I 
watch another, it seems to be just over the limit to record 3 while I 
watch one, and it definitely can't commflag a recording while 
recording two and watching another - I rather suspect I'm hitting the 
disk I/O limit of the single drive.

On the whole, I reckon that you are likely to hit disk I/O limits 
long before you hit the Dom0 network bottleneck.

At some point I'll be looking to upgrade and I've been thinking about 
how to arrange things. I could just do what I've got now, but with 
better hardware; or I'd considered running Myth on Dom0 and the rest 
as they are now.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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