[mythtv-users] How could logging cause myth performance issues?

[Jerry Rubinow]

On Wed, Jan 20, 2010 at 10:29 PM, sonofzev at iinet.net.au <
sonofzev at iinet.net.au> wrote:

>
>
> On Wed Jan 20 22:20 , Jerry Rubinow  sent:
>
> >Since switching to 0.22, I'd been having frame drops in the recorded files
> (problem was definitely in the recording, not the playback), delays in
> frontend
> activity.  The frame drops coincided with frontend activity while recording
> (playing back files, deleting files).  Network showed no errors.  CPU usage
> on
> backend remained low (much less than 50%) throughout.  Upgrading from
> weekly
> 0.22-fixes to trunk had no effect on the issue.
> >
> >Long story short, turning off all logging except for "important" messages
> caused
> all issues to go away.  The logging options had been the mythbuntu default
> (important,general,record,channel).  Turning on more logging options made
> the
> issues worse.  Size of log file varied per day between 15MB and 30MB.  This
> doesn't seem like enough throughput for disk i/o to cause network packets
> to fall
> off the queue.  The disk has dma enabled, and the recording storage is on a
> different drive than the system drive.
> >
> >Does anyone have any ideas what I might look into to figure out why
> logging
> triggered these problems?
> >Myth setup:Revo frontendAthlon 64 3500 backend, with HDHomeRun and HD-PVR
> tuners
> >Mythbuntu 0.22 fixes weekly builds (reverted after trunk didn't help) with
> Ubuntu 9.10
> >Before upgrading to 0.22, I was running 0.21-fixes on an older computer
> (Asus
> A7N8X-X) no issues.  I upgrade to 0.22-fixes and added the HD-PVR, and
> started
> having problems.  Switched the BE onto the Athlon 64 box, problems didn't
> change.
>  Problems occurred whether or not I was using the HD-PVR.  I believe (but I
> can't
> swear) I had the same logging options turned on when I was running 0.21.
> >
> >Thanks for any ideas.
> >-Jerry
>
> Not sure about your particular scenario.. But logging increases disk-write
> activity ... Working in call-delivery systems (call centre applications) we
> generally recommend to our customers they have logging kept to a minimum
> unless
> we are looking for the cause of a fault...
>
> If you have logging turned on, it's best that it isn't on the same disk as
> your
> Myth Database..
>

Is myth's mysql activity that intense?  hdparm reports that the
system/mysql/logging drive does timed cache reads at 862 MB/sec, and
buffered disk reads at 64 MB/sec.  dd can write 58MB/sec.  I would have
thought lots of small writes from the logging would be buffered and wouldn't
be enough to overwhelm the disk io.
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