[mythtv-users] Highest number of simultaneous streams recorded?

[Brandon Beattie]

On Thu, Oct 13, 2005 at 06:14:23PM -0500, Anand_Inala at Dell.com wrote:
> What is the most number of streams anyone on the list has simultaneously
> recorded without problem?  I assume recording 2 streams while watching a
> third has been done, but has it been done with no loss in quality, etc?
> How about 3 streams?  HD + an analog stream?  I'd like to get an idea of
> what my limitations and potentials are before I get too deep in my
> current project.

The current limitations on number of streams has to do with what
hardware you choose to use.  This includes tuner cards, hard drives,
network cards, and CPU.  I think it would still be rather easy to get
10+ streams recording and 4-5 being played back (1 local, 3-4 remote)
before you see any problems.  To do this you would need either hardware
assisted analog encoders, or an HD tuner because they won't use more than
3% or so CPU.  To reach a 10Rec 5Play number, you would want a good
processor and memory, something 3.4Ghz or over would be fine -- If
you're not going to watch video locally though, I bet you could do all
this with 2Ghz or less.  Disk usage is the next issue.  Using raid
0, 5 or 10 would help in this areas you may be able to do 15 streams
total with 2-3 striped drives I would bet.  Networking will be the final
issue.  HD streams run up to just under 20Mb/s.  As much as we wish to
get 1Gb/s speeds all the time, expecting much over 400Mb/s constant is
not always possible.  Myth struggles to play video smoothly unless it
feels like it has room to breath and almost no packet loss.

Personally I've recorded 5 HD streams at once (3 pcHDTV HD-3000's and 2
pcHDTV HD-2000 tuners) and watched 1 at once.  The load on the (backend)
AMD 2500XP was about 10% (Viewing the single stream to a frontend, P4
3.0Ghz).  I've recorded 4 full HD station feeds (One channel was
broadcasting 43Mb/s of data so I recorded this stations feed 4 times and
saved the entire feed as-is to disk) to a single drive.  This was all
done prior to Myth getting an internal memory buffer that now only
write disk when it needed to, not every second.  When this buffer was
added, the overall throughput to disk went up 3x-5x times.  However, if
I peg a 6 disk setup with dd'ing /dev/zero to a file, I can interupt a
single recording disk write and lose some data - Ths fix for this though
is simply increasing the memory buffer in Myth for recording HD data.

Right now the only limitation I see is how many PCI slots your computer
has, how many firewire devices you can reach before hitting the max
bandwidth for the motherboards firewire bus, and how much data you can
push through a network client to remote frontends.  Right about when you
max these things out I think the CPU and system bus would be near maxed
and that's the point where Myth would start to struggle.  At this point,
adding extra backends will let you scale on a new level, but I've never
experimented in this area.

--Brandon