[mythtv-users] Opinion on a P4 Backend // HDD Throughput

[Greg Mitchell]

Josh White wrote:
> 
>     When recording, Myth inserts stuff into the database like crazy, which
>     results in a lot of head thrashing (write stream one, write to database,
>     write stream two, write to database)  Throughput on the drives can be
>     fine, but as soon as you're causing constant seeking, the system
>     starts to
>     suffer.
> 
>     Greg
> 
> 
> The OP asked if his hardware would handle 4 tuners/4 frontends.  
> Yesterday, I was able to watch 3 different things, record 5 different 
> things, commercial flag 2 of the 5 recordings - simultaneously, on 7 
> year old hardware.  I was recording to 1 LVM comprised of 2 ATA 133 hard 
> drives.  It seems to me that if that won't push it too far, then I don't 
> know what will. 
> 
> I think we need a better definition of "like crazy" and "a lot" before 
> we build systems around it.

Suffer means - if I was trying to record HD and watch it on a time delay 
while an SD recording was going on, the PVR 250 was filling it's 
internal buffers, and I dropped frames.  Then the ivtv driver would 
eventually stop responding requiring a reboot before doing another 
recording with it.  The best part was that ivtv wouldn't stop responding 
for a couple of hours after this scenario happened, and it would 
sometimes be 12 hours before I noticed that it had stopped recording.

I don't know how your system is set up, but I guarantee that mine would 
have had a heart attack under your scenario.  Since I moved my 
recordings onto a separate drive from Mysql/the rest of my system, it's 
been rock solid.

Granted, I'm also doing some other stuff on the box that may have 
contributed to more thrashing - web server mail server etc, but it's all 
pretty low volume.

Probably contributed to my problems, as Apache would have been logging 
stuff, hitting mysql for data etc. as well.

Have a look at recordedseek - mine has about 800000 rows for about 200 
recordings.  It's not huge data volumes, but it's a largish volume of 
transactions which will cause the drive head to have to reposition every 
time they're flushed to disk.

The system /should/ be able to handle it, but from my experience, if at 
all humanly possible, don't put MySQL and your recordings on the same 
disk.  Bad things can (and have) happened.

As soon as I separated them out onto a new drive, I have experienced a 
new level of system stability.  I went from requiring at least weekly 
reboots (and accompanying lost recordings) to uptime that can be 
measured in months.  That's worth mentioning when someone's asking about 
HDD bandwidth.

Greg