[mythtvnz] Recording two channels from one multiplex

[Stephen Worthington]

On Thu, 21 Jun 2012 18:05:42 +1200, you wrote:

>On Thu, Jun 21, 2012 at 4:32 PM, Worik Stanton <worik.stanton at gmail.com>wrote:
>
>> On 21/06/12 16:22, Paul Kendall wrote:
>>
>>> So I should be able to record all the channels from the multiplex
>>>> >  simultaneously.
>>>> >
>>>> >  Do I have to do anything to set that up?  How do I find out how?
>>>>
>>> On the backend setup (mythtv-setup) you need to set the number of virtual
>>> tuners
>>> to the number of simultaneous recordings you need from a single multiplex.
>>>
>> Thanks.   That is very helpful
>>
>> But, err..., how many recordings do I need from a single multiplex?  I
>> want them all.  I have eleven channels on mplexid==6, but I think that is
>> an error
>
>
>Do you ever record from all 11 channel simultaneously? I guessing not.
>
>Set the number of virtual tuners to the number of simultaneous recordings
>you want to do from a single multiplex (keeping in mind any overlap that
>could be caused by starting recordings early or ending them late). The
>number of recordings you can do will be limited by your system,
>particularly disk access and possibly the database. You might have to
>experiment to see what your system can handle. But I doubt you'd have any
>problems doing 4 or 5 simultaneous recordings and that is likely to be more
>than enough.
>
>Cheers,
>Steve

You can set as many virtual tuners as there are channels on a
multiplex if you like.  There does not seem to be any limiting factor
in the tuner code itself.  The limits come with your recording drives
- if you have too many programs recording on one drive, you will get
missing bits in one or more of them when the RAM buffers overflow
before the data can be written to disk.  I have three DVB-T tuners and
I have each of them set to 4 virtual tuners.  Until ChoiceTV came
along, that seems to have been enough, but I am thinking of increasing
it to 5 each.  However, I now have 4 recording drives, so I can easily
cope with many programs recording at once.  When working out what your
drives can cope with, you need to account for the overlap on pre-roll
and post-roll, and also if you are doing commercial scanning then add
on one more process reading a file for each commercial scanning (or
transcoding) job.  So if you are recording 3 programs each ending at
20:30 and another 3 starting at 20:30, you will actually be recording
6 programs at once from say 20:29 (pre-roll of 1 minute) until say
20:34 (post-roll of 4 minutes).  Plus say two more commercial scanning
processes running, for a total of 8 streams of data going on and off
disk.  Then add one more for any program you are playing in
mythfrontend at the same time.

Also, if the drive the system and database is on is used as a
recording drive too, you need to account for heavy use of the
mythconverg database every time the scheduler runs, which is at the
beginning and end of programs in particular.  It normally only takes a
second or two, but it can really hammer the disk usage.  Mythbackend
is pretty good about spreading the usage around multiple drives, and
has an option somewhere to tell it the strategy you want it to use.  I
have that set to "Combination", which takes into account all relevant
factors.  I would recommend to anyone that they have at least two
recording drives (with the system and database on one of them).  And
recording drives are best to be 7200 rpm or AV rated.