[mythtv-users] backend 4x pvr250 MB advice

[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*

A big fast raid setup -software or hardware- can already just about saturate 
the PCI bus IF the diskcontrollers are attached to the PCI bus. The same 
certainly goes for GigE cards.  Add to this the four PVRs, and keep the 
latency in mind that all that bus-sharing entails.  I'm always being 
cautious; just like you'll never get 12 Mbyte/s out of a 100Mbit NIC, I must 
assume the practical PCI bandwidth is much lower than 133 MByte/s.

> >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).

Can't comment on the two backends setup- and maintainability issues since I've 
never done it.  But taking the price of the 4 PVR250s and the disks into 
account, there is not much price difference overall between one high-end 
system and two low-end ones, especially if it's decommisioned PCs.
You'd have to connect the two backends over GigE, preferably over a crosscable 
but I'm not sure MythTVs' network topology allows for that.
Apparently, sharing a SCSI bus is the easiest way to make your systems 
unstable so I'd certainly advice strongly against that...

> 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.

> >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.

> 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 ;^)

I hope, for his sake, that isn't the case.  I have just one TVcard but I often 
lack the time to view it all back.  Now with 4 cards...!   ;-)

Maarten

-- 
Linux: Because rebooting is for adding hardware.