[mythtv-users] restricting mythpreviewgen?

[Alex Williamson]

On Wed, Jan 23, 2013 at 8:30 AM, Mike Perkins
<mikep at randomtraveller.org.uk> wrote:
> On 23/01/13 14:59, Alex Williamson wrote:
>>
>> On Tue, Jan 22, 2013 at 11:48 AM, Alex Williamson
>> <alex.l.williamson at gmail.com> wrote:
>>>
>>> On Tue, Jan 22, 2013 at 11:19 AM, Raymond Wagner <raymond at wagnerrp.com>
>>> wrote:
>>>>
>>>> On 1/22/2013 12:38, Alex Williamson wrote:
>>>>>
>>>>> I can restrict other jobs to not run on this slave, can or can I not
>>>>>   restrict mythpreviewgen?
>>>>
>>>>
>>>>
>>>> Mythpreviewgen is not a job that runs through the jobqueue, and thus
>>>> cannot
>>>> be restricted to a single host in the manner that the other tasks can.
>>>> It is
>>>> always run by the backend that owns that recording. Assuming your slave
>>>> backend has no storage of its own, and records to shared storage on your
>>>> file server/master backend, your only real option is to have something
>>>> run
>>>> after recording is finished to change ownership of that recording to the
>>>> master backend.
>>>
>>>
>>> Now we're getting somewhere, thank you.  A mysql cron script wouldn't
>>> be too hard.  Alternatively I was thinking about replacing
>>> /usr/bin/mythpreviewgen on the slave with a script that just does an
>>> rsh of the job to another system.  Your idea might be less tedious to
>>> maintain though.  Thanks,
>>
>>
>> This does seem to work, if I update the database like:
>>
>> mysql> update recorded set hostname="<master>" where hostname="<slave>";
>>
>> Then when I load mythweb I see mythpreviewgen running on the master.
>> Next step is to figure out when it's safe to do this.  Can I do this
>> while the recording is still going?  If so I could fork a job from the
>> hdpvr channel change script to sleep briefly then update the database.
>>   Alternatively I could just create a user job to update the database,
>> with the downside being that previewgen during recording still runs on
>> the slave.  But I could link mythpreviewgen to /bin/false on the slave
>> and just not get previews until after the job runs.
>>
> I would guess that if you linked mythpreviewgen to /bin/false the slave
> would think it failed and just keep re-spawning it. Better to link it to
> /bin/true and have the slave think the job finished ok.

Good point

> You might not need a cron script. Can you use mythtv events to trigger off
> the previewgen where you want it?

I've never looked at the events interface, perhaps so.  By user job
I'm referring to the myth controlled job launcher, not cron.

> Also: There are a lot of other tables in mythconverg which have a 'hostname'
> column. Some of those /might/ need updating too.

Yep, a more thorough search of tables is in order, but it seems
logical that recorded.hostname is sufficient since all the files have
the same path on a shared nfs automount.  Thanks,

Alex