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