AW: [mythtv-users] Large-scale myth farm

Martin Bene martin.bene at icomedias.com
Mon Oct 3 16:42:04 UTC 2005


HI Rob,

> We'd only be recording some programmes. We've enough experience of
> enterprise systems to build a resilient backend (though we'd have
> to look at how we'd do failover on the master backend - the SPoF
> of the system). How many quad opterons (Sun do the nice v40z)
> would it take to transcode say four programmes simultaneously?
> Could one server handle 4 DVB capture cards? What's the first
> bottleneck people hit - the PCI bus? hard drive speed?

My current system has 4 DVB-S cards, no problems at all with 4
simultaneous recordings. Single P4 2.4Ghz. If you want to keep these
recordings as references copies I'd advice you not to transcode; after
all you do lose quality in the process, and the mpeg2 streams you get
off a DVB card are already in a reasonable format for archiving. 

I'm currently using 3 different models of cards in my system:

2x Terratec Cinergy 1200 DVB-S: basic card without support for CI
modules. 
1x Hauppauge Nova-CI-S: used with cryptoworks CI module and ORF smart
card for austrian programs.
1x Hauppauge Nexus-S: used with  premiere CI Module and smartcard.

Without requirement for CI modules I'd probably stick with the terratec
cards; small, don't run too hot and no problems so far.

Also, the dvb mpeg2 recordings can contain multiple audio streams
(multiple languages, different formats) - these will be recorded just
fine in mpeg2 format. Transcoding usually removes all but one of these
audio streams.

So, where can you expect bottelnecks to appear in your system?

 - Cooling/Power Supply: Capture Cards (including DVB) create quite a
significant thermal load; shouldn't be an issue with real server class
hardware, but can fry badly cooled systems, especially small-form-factor
stuff.

 - PCI Bandwidth: the DVB Recordings I get are somewhere in the
5-8Mbit/s range. That'll give you 32Mbit Input for 4 cards; take x2
because you'll probably want to store it somewhere as well. Still
comfortably below the bandwidth of a single PCI bus.

 - Storage Subsystem: at ~ 1Mbyte/s per recording certyinly no problem
for local storage and 4 Cards; 30 channels on a shared storage server
should also be possible with a bit of planning. 

Bye, Martin


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