[mythtv-users] Backend for Multiple HD/Firewire streams

[Brian Foddy]

On Wednesday 09 March 2005 06:51 am, Pat Mac wrote:
> I'm planning to convert my current p3 1 Ghz standard definition
> backend into a firewire/hd backend (ATSC) and I just want to make sure
> my thinking is correct.  I don't want to convert this and then find
> out I was off by a factor of 10 and my PCI bus is saturated.
>
> 1.  Firewire & OTA streams come in at a max bitrate of 19.3 Mb or ~2.4
> MB per second
> 2.  Without gigabit ethernet, I can connect at max 4 frontends
> 3.  With multiple frontends I'm probably better off throwing a RAID in
> the machine as well to maximize disk speed.
> 4.  Even with 4 streams coming in, 4 streams going out, and a hardware
> RAID on the same PCI bus I'm not even close to maxing out the PCI bus
> because I'm streaming in at ~10MB/sec, writing ~10 MB/sec to disk,
> reading ~10MB/sec from disk, and writing ~10MB/sec to ethernet leaving
> me with ~93 MB/sec still available on the bus and nothing to worry
> about.
> 5.  Because the ATSC streams arrive pre-compressed in MPEG2, my system
> will have no problem handling 4 of them since there is very little
> processing going on
>
> Am I missing anything major here before I start?
> Also, if anyone has Time Warner with a 3250, does the TW OSD show up
> in the video from the firewire port?  Can you set the output format
> (say, send every chanel at 480p)?
>
> Thanks to everyone for the great system.
> --Pat
> Long time listener, first time caller.

Be careful of the network bandwidth.  I was playing around this weekend with
multiple slave backend configs.  A 100mbps ethernet fell short doing:
writing 3 - pvr250 records, 1 air2pc HD recording, and playing a HD recording
on a remote NFS filesystem.  If you simply add up the streams together, it
might fit, but NFS is not efficient enough.  I even multiplied the read and 
write buffering in myth by 10, used 8192 blocks, and nfs ver 3, but it wasn't
quite enough.  My network monitors showed 12-14 MBps (bytes), so I
was swamping the network.

So assuming the 4 hd cards are local to disk, you would still
fall short streaming 4 hd shows out to frontends at the same time at 100mbps.
I'm ording a gigabit switch right now.

Keep in mind, both file system caching and NFS caching seem to bunch
updates in bursts.  If your frontend needs a read during that burst,
it may have to wait.