[mythtvnz] Recording dropouts and disk performance...

[Stephen Worthington]

On Sun, 17 Feb 2013 10:10:38 +1300, you wrote:

>On Sat, Feb 16, 2013 at 08:36:45PM +1300, Robin Gilks wrote:
>> Hi Rob
>> 
>> You don't say what filesystem you are running or how much free space you
>> have available. Also, what space have you asked MythTV to keep available
>> as a 'float' so it can record without immediately deleting stuff. Also, do
>> you have slow deletes enabled (only useful on some filesystems).
>> 
>
>Oops, sorry - the filesystem is ext4. Not sure about the float or slow
>deletes settings, but I don't remember changing them from the defaults.

Ext4 is not on the recommended list for a recordings partition,
although quite a lot of people do seem to be using it successfully.
But you definitely need to check that the slow deletes option is
switched on with ext4.  The recommended filesystems are JFS and XFS:

  http://www.mythtv.org/docs/mythtv-HOWTO-3.html

I use JFS.

If you are using ext4 with slow deletes off, then whenever you are
watching a program and finish it and delete it, if you are recording
at the same time you may have performance problems.  And when your
drives get full and autoexpire is happening at the same time as a
recording, then the same problem will happen.

The thing you have to remember about modern hard drives is that
although they have much, much faster read and write speeds, the
stepping speeds have not changed.  So every time the heads have to
move a long distance, there is a dramatic delay.  Deleting in an ext
filesystem seems to involve lots of head movement.  So if your heads
are already moving back and forth between two recording files that are
in widely separated locations, then adding the extra head movements
for the delete can cause too much delay.