[mythtv-users] backend 4x pvr250 MB advice

[Stephen Tait]

>
>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*

>You need to experiment, I would not expect a system to be able to
>maintain 4 recording streams and 7 frontend streams at one time, albeit that
>worst-case scenario will not happen often at all...
>Maybe splitting the backend into two separate boxes with Gig-E between them
>and two cards in both will work better, I dunno.  That would at least help
>with the network bandwidth, as not all frontend streams will be served by the
>same backend.

Two backends would obviously solve the limits of a particular machine, but 
of course it's going to be a) more expensive b) more difficult to maintain 
and c) more difficult to access resources (such as shows) on backend 1 from 
backend 2 and vice versa, and you're right back to the PCI/network 
bandwidth again (unless you do something funky like SCSI RAID cards chained 
directly together on separate machines or something).

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.

>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.

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 ;^)