[mythtv-users] how many streams can a single HD sustain?

[Raphael Pooser]

I have a question about just how much you can do under myth with just 
one HD.  If you have a 7200RPM ata133 HD assuming DMA is working right, 
it seems that you can really do a lot.  For example I read a review that 
says if you have enough processing power you can record and software 
encode, watch a previously recorded show (decode) and transcode another 
separate recording all at the same time.  It's perfectly feasible that a 
mid range processor could handle all this load, but how much could a 
typical hard drive really take?  To record and watch at the same time, 
you need to be simultaneously reading and writing two separate streams.  
To transcode something on the same hard drive you need to be able to 
read and write another separate stream.  So, in summary you have three 
steams, one both read and write going on, one reading, and another 
writing, all simultaneously.  Question is, what is the limit of a 
typical HD?  After what amount of load are the tasks going to start 
becoming difficult and the machine begin to have problems with 
playback?  It ocurs to me that this is totally different than having any 
number of front ends reading the same recording from a backend, because 
the backend only needs to do one read to stream the data to all 
frontends.  But what if you keep increasing the number of frontends 
connected to a backend, each asking to watch a different recording, so 
that now multiple streams are open?  Which becomes the bottleneck first, 
the processor or HD (ignoring network bandwidth being a problem)?
One thing that would be useful is if myth could use separate hard drives 
for separate tuners if multiple streams became a problem.
Raphael