[mythtvnz] Pixellation Every ~ 20 minutes

[Stephen Worthington]

On Fri, 5 Oct 2012 19:18:41 +1300, you wrote:

>On Fri, Oct 5, 2012 at 2:56 PM, Stephen Worthington
><stephen_agent at jsw.gen.nz> wrote:
>> On Fri, 5 Oct 2012 12:15:07 +1300, you wrote:
>>
>>>ThreadedFileWriter.cpp:499 (DiskLoop) -
>>>TFW(/mnt/media/mythtv/4301_20120926082000.mpg:56): write(61100) cnt 15
>>>total 818552 -- took a long time, 1663 ms
>>
>> I would say this is the likely culprit - 1.6 seconds is way too long
>> for a disk write operation, so it is likely some data was lost at this
>> point.  But just what caused it does not seem to be shown in the log.
>> You seem to have only been doing the one recording at the time, so
>> there should not have been any issues with too much disk access.
>>
>> However, your recordings drive is ext4, which is not actually
>> recommended for a recording drive.  It is so long now since I set up
>> my system that I can not remember why ext partitions are not
>> recommended, but I do know that ext3 and ext4 were not.
>
>I think it is the high disk usage and long time taken deleting a large
>file (mythtv recordings are large files in this context, and the
>system could suffer from the delays.
>
>Come to think of it look for a file delete (auto expiry?) happening at
>the same time as the glitch - maybe your drive is real full and at
>about 20 mins into a recording myth decides to delete an old recording
>and disk usage goes thru the roof?

Yes, that was the problem.  Ext filesystems have a horrible delete
process that practically locks up the drive until it completes.  It is
only usually a problem with larger files, but that includes almost all
video/recording files.  It also happens when a large log file is
deleted, which happens from cron jobs.  There is no workaround - you
have to use a different filesystem.  If there are multiple deletions
coming from MythTV, there is an option somewhere for "slow deletes" or
something like that, which I think prevents back-to-back deletes from
making the problem even worse.  You can make sure that your manual
deletes are only done while not recording, but automatic ones will
cause problems.  Automatic deletion of live TV recordings, and when
you have a program set to record only a set number of episodes and it
starts recording a new one, those will catch you out.  And when your
drive is full and it autoexpires things - it checks for free space
around every 15 minutes and does expiries until there is enough free
space again.