[mythtv] Re: New idea for storing recordings to disks

[Joseph A. Caputo]

On Wednesday 29 June 2005 10:33, Robert Tsai wrote:
> On Wed, Jun 29, 2005 at 09:50:29AM -0400, Joseph A. Caputo wrote:
> > > The point is that for MOST recording methods, Myth can't known in
> > > advance whether a recording will fit in a given location. Only
> > > PVR-x50 recordings seem to have predictable file size - DVB, ATSC,
> > > RTJPEG etc do not. 
> > 
> > But that's the case currently, anyway.  As it is now, we only have a
> > setting that controls the minimum amount of free space that must be
> > available before Myth will start a new recording.  There's no
> > guarantee that there is actually enough space to *finish* the
> > recording... you just have to set the number high enough and hope
> > for the best.
> 
> Doesn't auto-expire run every <n> minutes? It seems that if free space
> dips below this threshold during the making of a recording, then
> auto-expire should kick in to free up some more space.
> 
> If your bitrate is wildly variable, then it seems like the solution is
> to simply decrease <n> (e.g., have auto-expire make more frequent
> checks).
> 
> It also means that Myth doesn't need to know the eventual size of the
> recording, since it's checking for free space every so often anyway.

All well and good, but there are still special cases and considerations:

(1) if your auto-expire space threshold <t> is not at least as large as 
the max number of bytes a recording could consume in <n>, you could 
still run out of space (not too likely, but possible)
(2) if auto-expire runs, but there's nothing that can be expired, you're 
hosed

#1 can be mitigated by making sure your auto-expire space threshold will 
leave enough space for at least <n> minutes of recording (possibly <n> 
* <number of potential simultaneous recordings> + <fudge factor> ?).

#1 & #2 can be mitigated by setting the min. new recording space 
threshold large enough to hold the largest anticipated single recording 
(times number of tuners).

Bottom line is still, to *guarantee* you won't run out of space 
mid-recording, you must set the new recording space threshold high 
enough, figuring in factors like multiple tuners and transcoding jobs 
that may also consume space.

-JAC