[mythtv-users] System Load/Performance Question

[Brian Wood]

I know the "can this system do <blank> has been done to death here,  
but I need some comparisons.

The system is an amd64 3700, running Gentoo 2.6.15, MythTV  
0.18.1,1.5GB RAM

We are talking strictly about SDTV here.
2-PVR-150s
GeForce-5700 video card w/256MB.
nVidia 7676 drivers, no XvMC, MythTV compiled with OpenGL support

Recordings are going onto 2-250GB SATA drives configured Linux kernel  
software RAID0, JFS filesystem.
Ring Buffer is going onto a UDMA 133 250GB single drive, JFS  
filesystem, no other drive on the IDE channel, The OS is on another  
partition of this drive.

If I have a recording happening on tuner 1, and am watching LiveTV  
from tuner 2, and have a commercial flagging job happening, I get  
digital breakup on the live picture, and occasional breakup in the  
recording.

Without a flagging job running there is no breakup on liveTV or the  
recording.

CPU usage shows about 30-35%, no swapping is happening.
The priority of the flagging job in MythSetup is set to "low".
vmstat shows "WA" as "1", and very occasionally "2"

So, my questions:

Is this normal, am I simply asking too much of the system?
Is it a disk IO problem? (I don't think it's a CPU problem)
Would putting the ringbuffer on the RAID system help?
Would moving to a SCSI disk system help?
Should I just run flagging jobs at other times?
Is having the ringbuffer on the same drive as the OS a bad idea?

Obviously I'd like to figure out what the weak link is in this system  
that is causing the breakup, any ideas appreciated.

Brian Wood
beww at beww.org