[mythtv-users] backend 4x pvr250 MB advice

[Chris Pinkham]

> Couple of points to the OP: 
> You either will need gigabit ethernet, or several 100mbit cards in the backend 
> to serve frontends I think.  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.  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...

These streams are more than likely less than 1 Megabyte per second, so this
isn't that big of a deal for a server.  If you record at 4Mbit/sec, that's only
500Kbytes/sec so that's even less.  You'd have to be recording at 8Mbit/sec to
have each stream pushing 1Mbyte/sec (~3.6GB/hour).  You have to remember since
the pvr-x50 and M179 cards use hardware compression, their impact on the PCI
bus is miminal even the highest bitrates since they're only copying around
compressed data.

> 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.  This setup of yours is kind of uncharted territory of course, 
> and that is obviously why you're asking here.

My master backend is running 3 M179 cards on a P2-400 Dell PowerEdge server.
All storage (except the OS) is on NFS at 100Mbit.  The only problems that I
see are occasionally when I'm recording 3 things at one time as well as
commercial flagging a few others and watching something else, my NFS
server can't keep up since it only has 1 hard drive for video right now
so the heads are being beaten to death because of the 7 (or more) streams
being read/written.  The p2-400 chugs along fine and bandwidth isn't a
problem even though all 7 streams are going across the network.

I believe the write buffer code in Myth calls fsync() (or fdatasync()
if available) every second as long as data is being written, so that can
cause quite a bit of head seeking.

-- 

Chris