[mythtv-users] Question about Mythbackend

Bruce Markey bjm at lvcm.com
Mon Mar 31 21:10:59 UTC 2003

Joseph A. Caputo wrote:
> What do you mean by a "mythbackend box"?
> Any box with a tuner card that you want to use with Myth needs to run
> mythbackend, and needs to save the data to its own disk.  A network file
> system (NFS, Samba, etc) will not provide enough throughput (regardless of
> how fast your network is).

I agree with most of this very helpful response but this isn't
exactly true. Writing to a local disk is certainly preferable
but if the recordings are, say, 1.5GB per hour that is still
less than 1/2 MB per second. This won't work over a wireless
network, questionable for a 10Mb network and won't work for
more than one recording at a time. However, a 100Mb net would
work and, in fact, there are people out there using NFS. The
problems with multiple NFS recordings would likely be latency
rather than throughput. With four recordings over NFS that's
still less than 2MB/sec but with four connections competing
to deliver packets to the server and the disk heads thrashing
to write four files, the encoders would probably be dropping
frames as they couldn't write their data in time.

>  In your case, you probably will want to set up
> one box w/tuner as the master backend and the other 3 as slave backends.
> Myth will take care of integrating the files on the different boxes into a
> single 'view' for the frontend.  Any, all or none of your backends can also
> be frontends at the same time, as long as the box is fast enough to handle
> simultaneous encoding (record) and decoding (playback/LiveTV).  My advice is
> choose your best tuner card & put it in a powerful box with the biggest disk
> you have.  Set that box as the master backend, then configure the other
> boxes as slave backends.

Good explanation. I agree. Currently, the tuners are assigned
in order so 1 is always first. 2 is only used when 1 is busy
(live TV or recording) etc. Therefore the master should have
the biggest disk (and the best tuner ;-).

> Another option is to put multiple tuner cards in a single backend.  This
> ability is only limited by the ability of your box to record/encode multiple
> programs simultaneously, and the availability of sound inputs on that box.

Right but you may not want to use more than two tuners in a
singles server.

It used to be that the scheduler knew how to use one or two
tuners. With the multi-backend, it can now use any number of
tuners no matter where they are. Therefore, leading up to the
0.8 release I tested three tuners on one machine and it 'worked'.
I know that four would 'work' but this is kind of a pointless

To do medium to good quality recordings with mpeg4 you need
about 1GHz per tuner. The parameters have to be set low enough
so that it will work when all tuners are busy. This means that
if you got a bleeding edge >3Ghz CPU and several tuners, it
could only give you medium to low res recordings and not make
use of it's resources the vast majority of the time.

You may want to consider two dual servers for your four tuner
cards. Local disks on each of the servers then lightweight,
diskless systems for your four frontends.

Again, the first card is always used first so the master
needs more disk space. However, if you modify the database
so that the card order is master=1&4, slave=2&3 the disk
usage should be more evenly distributed.

Back to the original question ;-), I haven't tried it but
a backend should be able to work without a graphics card.
However, as Chris Palmer pointed out earlier, the backend
"setup" program needs X so you would have to install the
X packages and remote X authentication just to go through
the setup program. After it is installed, it should no
longer need X (at least until the next time you go to change
something in setup).

No audio card can only work if btaudio works for your cards.
AFAIK, there is not make/model that is sure to always work
much less work and have good sound quality.

--  bjm

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