[mythtv-users] backend 4x pvr250 MB advice

[Steven]

Op vr 30-07-2004, om 20:05 schreef Maarten:
> On Friday 30 July 2004 19:06, Stephen Tait wrote:
> > >Couple of points to the OP:
> > >You either will need gigabit ethernet, or several 100mbit cards in the
> > >backend
> > >to serve frontends I think.
> >
> > I think gigeth is a must. Assuming the MPEG2 stream is 12Mbit-ish, 7
> > simultaneous frontends connecting to it would easily swallow the bandwidth
> > on a 100Mbit card. Using several NIC's is probably a cheaper option (i.e.
> > you don't need a gigabit switch) but I've never had a huge amount of luck
> > with it myself (plus I picked up a dirt cheap 2*1000Mbit and 8*100Mbit
> > switch, so never tried that hard).
> >
> > >It's rather dubious if 7 frontends can be driven
> > >though just one network card. The issue of disk I/O and PCI bandwidth
> > > plays, too.
> >
> > In case you didn't know already, PCI bus is 133MB/s. But I don't know how
> > much traffic PVR streams generate on the bus (is it not just an MPEG2
> > splurge onto the HD, plus control traffic?), not how much concurrent
> > streams would interfere with one another (and a RAID card). *goes to scour
> > the ivtv tech docs*

I'll be using gigeth for the backend since there is a gigeth switch and
cabling troughout the house. But it shouldn't really be nesccessary even
playing back 4 live-tv streams (bitrate 8000) would only be 4,5MB/sec.
If you add 3 recorded streams to that you'll end up serving 6,5-7 MB/sec
and that would still be ok for 100 Mbit but getting close to the safe
limits.



> > I mentioned it casually before, but am still wondering about the TV card in
> > the diskless frontend - is it even possible? If it is, I'm assuming that
> > the live TV would have to be fed to the backend, and then back to the
> > frontend, over the network. Argh, goodbye bandwidth. You could probably
> > finagle the buffer to write to a RAMdisc, but then obviously you'd need
> > gobs of memory to allow for long pauses. Argh! Scrap that idea.
> 
> Um, a frontend with a TVcard is just another backend.  So it needs to NFS 
> mount the share directory off the master backend.  So one needs to understand 
> how NFS data is cached of the clients.  It is conceivable that the live TV 
> stream -that was just written seconds ago- is still cached in ram on the 
> diskless machine.  But I can only guess here.  I don't even know how you read 
> or write from a ringbuffer, that's probably diffferent than from a regular 
> file.
> 

I've set up a diskless EPIA station netbooting as a slave backend last
night. Live tv works (the card in the backend was recording so I'm sure
i was using the slave card) but I'll have to do some testing with
bandwith usage/buffering. I'll post results here. 
But anyway I think worst case scen. will have to be taken into account
where one is behind live-tv by a few minutes. With 8000 mbit live tv
recording and playing back we'll only be looking at about 2 Megabyte/sec
network traffic.

> > >This setup of yours is kind of uncharted territory of course,
> > >and that is obviously why you're asking here.
> >
> > Perhaps it'd help the diagnosis if we knew what sort of uncharted territory
> > we were attempting to terrorise? If the system is designed to serve
> > multiple households, then I'd say there's a fairly good chance of all
> > frontends and tuners being in use at the same time. If it's just for one
> > (large) household with alot of TV's and not too many people, then the
> > chances of this are much less, and you can adjust the hardware accordingly.
> 
> Yeah, good point.  Six idle frontends don't pose any problem for a backend. 
> But seven active ones might.
> 
There shouldn't be more than 4 frontends working simul. unless they are
left on live-tv when no-one is there ...(and that is always a problem
for new, non technical, PVR users...)

> > My money's on the parent being a rich geek in silicon valley who works
> > hours that are far too long, and just wants to record all his TV whilst
> > he's away, and then he can flollop in front of the nearest TV when the
> > caffeine wears off, whilst leaving a tuner or two free for
> > family/friends/flatmates ;^)
> 
You're right exept for the location/continent :-) 
I'm not setting it up for myself tough. It's a 4 people household with 2
plasma screens, a projector(room) and 4 "ordinary" TV's. 

Thanks for all the usefull input so far.

Steven