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

[Josh White]

On Wed, Mar 5, 2008 at 9:34 PM, Greg Mitchell <greg at nodecam.com> wrote:

> 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
>

I agree completely with the separation of the recordings and the database to
separate physical drives.  I got lucky when I set my system up, and did that
by accident.  I guess I just assume that's how everyone does it (bad
assumption).

I haven't done anything too "fancy" with my set up, but I guess my myth
storage system does ultimately entail 2 computers and 5 hard drives. My main
backend has the 180GB LVM that I use just for recordings, and I have a 40GB
ATA 133 drive with my OS, database, etc. In addition, I have 1 TB I use just
for other media storage, like DVD rips, music, photos, etc. hosted on
another computer, but connected to the main backend via NFS (not that the
1TB drive in question was involved in my scenario).

This is my first mythbuild, and I didn't really read a lot on how to
configure this stuff, that just seemed like the best way to do it with the
hardware I had.  The only changes I've made since the beginning was to add
the 60GB drive to my 120GB drive using LVM, and I formatted my recordings
drive to XFS.

My system does not do HD at all (I have no HD displays, except perhaps if
you count my 1280x1024 monitor on my main desktop machine).  I've downloaded
and displayed HD content (720p h.264) on my main frontend, but it really
"suffered" on my P4 1.7ghz machine (pegs the CPU).   I was surprised to see
that it displayed on my SD TV, though I assume it did something to
down-convert.
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