[mythtv-users] Dropped frames, simultaneous recordings and process priorities.

[Stephen Worthington]

On Sun, 12 May 2013 21:40:46 -0500, you wrote:

>Our box is a 4 CPU 3.4GHz i5, 8G ram, Ubuntu 12.10, with a PCIe InfiniTV 
>4-tuner card (which produces mpeg-ts content). It usually has XBMC 
>running idle on it and mythtv. We also do background Handbrake encodings 
>and editing with Avidemux (as needed). I notice that under some 
>circumstances, our recordings have considerable noise, especially during 
>the first few minutes. My theory is that this occurs when consecutive 
>movies are recorded, or perhaps when more than one is recorded at once. 
>I do have the default set to record 1 minute before and after a show, so 
>we could be using two tuners and double the I/O for a minute for 
>consecutive recordings. On some occasions we might record two 
>consecutive shows on two channels simultaneously, so I guess we could 
>briefly have 4 recordings at once.
>
>After a recording is complete, I make a copy of the .mpg to the same 
>disk to simplify the task of my wife copying the file to her machine for 
>Avidemux (avoids symlink issues). I also create an Avidemux index file.
>
>I see that mythbackend runs with a normal *nice* value and with *ionice 
>idle*. I just modified my post-record script to run the copy and 
>avidemux create index with ionice idle and nice 10. Perhaps this will 
>solve my problem. But I am curious why mythbackend doesn't run with a 
>normal *ionice* value. Is the content from the InfiniTV considered I/O 
>by ionice and impacted by it? Does mythtv buffer frames as needed when 
>the I/O is backed up a bit?
>
>If I have to I'll rearrange the disks or acquire more or faster (SSD) 
>disks. But I'm not sure if *ionice**idle* requires ALL I/O to be idle, 
>or if it is on a per-device basis. Today I have a disk for the normal 
>Ubuntu stuff (root, /usr, /home, /tmp). I have one, /pvr, for mthtv and 
>a staging area for movies to be edited or encoded and one for movies 
>after they have been encoded for XBMC. They are not fast disks (5400)

You did not say how many hard drives you are recording to.  If you
only have the one hard drive with the system on it and are doing all
your recordings to that drive also, then yes, you are probably
overloading the drive.  Modern hard drives are very fast at sequential
writing, but when they have to move the heads back and forth between
four different recordings, plus the usual system accesses and the
occasionally heavy database use by MythTV (especially if the scheduler
runs), then they do get overloaded and blocks of recordings go
missing.  The head movement speed has not increased nearly as much as
the sequential write speeds have as drive technology has improved. The
fix is normally to add one or more extra hard drives to record to, and
move enough recording files over onto the new drives so that there is
reasonably equal spare space on all the recording partitions.

Also, it is very important to use JFS or XFS for recording partitions,
as deleting huge video files from EXT partitions frequently causes so
much activity that it will prevent recordings from working.  If you
are set up with an EXT type partition for recordings, then you can
help with that problem by setting the option for slow deletes.  Sorry,
I can not remember exactly where in the setup screens to find it.  If
your recording drives are full and having to expire old programs, that
can happen during recordings and on an EXT partition that will cause
problems without the slow deletes option.  Just one delete at the same
time as one recording on an EXT partition can cause problems.

Do you also run mythcommflag (commercial skip processing enabled)?  If
so, change its options in MythTV to use a lower priority also.  And as
you seem to have lots of RAM, you might like to set it to start at the
same time as the recording starts.  If you do that and have enough
RAM, mythcommflag will be getting all its data from the buffers in RAM
before the data is written to disk, so it will not be reading disk all
the time and moving the heads on the drives.