[mythtv-users] Is there a tuner limit? (problem with >8 tuners)

[Chris Wilkie - NetrinoUK]

Just to update everyone, I have solved the problem on advice from Robin
and Janne .... Thanks guys!

As suggested, I changed the value in
drivers/media/dvb/dvb-core/dvbdev.c, recompiled the modules, copied
everything into the right places and Linux was happy to detect all 6
cards (making 12 tuners).  Confirmed this with tzap - managed to tune
channels in on tuners 8-11 and see a tuning lock.

Next problem is that Myth's GUI doesn't let you select a DVB ID above 7,
so a bit of database hacking was required.  I just added four extra
cards on ID 7, then 'update capturecard set videoid=<8-11> where
cardid=<9-12>'.  The GUI isn't happy if you try to edit those cards, but
now it's all set up there's no need.

I'm also using the PAE kernel in order to make use of the 16GB available
RAM in the box.  The Nova-T 500 seems fine on this kernel (I was
worried, after reading about freezes/lockups on some kernels with this
card).

At this point everything was working fine, and I was able to make
recordings on all cards through Myth.

Now the problem I'm having is write speed on the disk array.  When
making 8+ recordings, the load average (and io wait) shoots up.  It
would appear that Linux is seeing the server's RAID system and switching
the software write cache off, except that this server (apparently)
doesn't have a write cache.  I guess performing 8+ random writes to disk
at 5-7MBit/sec each is causing the problem.  If anyone knows how to
force the software write cache back on (to save me some google time
finding out) please let me know! :)

Finally, to address a couple of points raised yesterday...

> Wow, more tuners than muxes, I but you wish the multi-rec branch had
landed?

The multi-rec brand looks a bit vague, certainly way over my level of
understanding.  I was also only able to find very little information
about whether it is stable, or even whether it works at all.  In this
application, the system just needs to work, so I didn't want to mess
around with development code.

To be honest, I'm throwing away most of the functionality of Myth
anyway, so I was considering just knocking up some Perl scripts to use
with DVBTools, meaning I could do multi recordings from a single tuner,
and doing away with Myth altogether.  All it needs to do is record
programs to disk from a schedule - I don't need any of the other
functions.  Maybe one day I'll get around to it, but for the moment Myth
is doing the job just fine and 12 tuners is plenty.  We're running fine
on 5 at the moment, but the recordings miss a lot of program start and
end points, as I'm not able to give it extra time at the start and end
of the shows.

>> FYI, hardware is an HP DL720 with 8x Xeon CPUs and 16GB RAM.

> Bet that's nice and quiet in the living room :-P

It's horribly noisey.  Lucky it's going into a data centre eventually -
I can't wait to get rid of it!

Thanks to all who assisted with this.

Cheers, Chris


-----Original Message-----
From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Robin Hill
Sent: 20 August 2007 14:42
To: mythtv-users at mythtv.org
Subject: Re: [mythtv-users] Is there a tuner limit? (problem with >8
tuners)

On Mon Aug 20, 2007 at 12:32:11PM +0100, Chris Wilkie - NetrinoUK wrote:

> Hi All
>  
> I've been pointed here by the chaps on the MythDora forum, hoping to 
> gain some advice on getting a box working with 12 tuners (6 x Nova-T 
> 500 dual tuner PCI cards).  I am able to get the first four cards 
> working, giving 8 tuners, but the last two throw errors when trying to

> load the driver.
>  
<- snip ->
> Now, 8 seems to be a convenient number to stop working on, so I'm 
> wondering if this is a limitation in the driver?  Or (hopefully) there

> is a tweak that needs to be done to allow this many tuners to be used.
> Surely I'm not the only one mad enough to run that many tuners in a 
> single box?
>  
Yes, 8 is a limit defined within the DVB subsystem
(drivers/media/dvb/dvb-core/dvbdev.c).  I suspect you may need to do a
little more than just modifying that value though - you're safest
checking on the linux-dvb mailing list
(http://www.linuxtv.org/lists.php).

Cheers,
        Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <myth at robinhill.me.uk>  |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |