[mythtv-users] Live TV playback frustration

[Adrian Saul]

On 19/08/2012 2:37 AM, Adrian Saul wrote:

> There is some lag on the playback startup and that occasionally 
> affects the other frontend too - so far the only thing in the logs is 
> time taking to do "SyncPositionMap" - which is odd because following 
> the code and doing SQL behind the query completes instantly when I 
> test it but in the logs I can see multiple instances where it takes 13 
> seconds or more.   Might take a closer look at what the database is 
> doing because the code shows nothing else that would block this.
>
So - for all my confidence in my infrastructure I took a look at the 
database to see what might cause this query to take so long.   Any time 
I manually tested it, the performance was fine - near instant response.  
When I took a look at the tables I realised some things:

- I have 1500+ recordings
- Those recordings have 9.1M rows in the "recordedseek" table which is 
queried by SyncPositionMap.
- Those tables were all on MyISAM tables from when I exported the 
database myself from Linux to Solaris.... doh

So while I could not reproduce it, I took a guess that table locking on 
a 9.1M row table was not going to be great under MyISAM, and started 
converting most of my tables to InnoDB and in the process tweaked the 
ZFS setup underneath it to match recommendations for InnoDB.   I had the 
memory for it so I also setup the database with a 512M InnoDB cache. 
That seemed to resolve the lagging start issue on the second frontend 
and also seems to make things a lot snappier in general.

I also revisited my NFS setup.  To try and fix the second frontend being 
unable to change channels (loses the jump buffer) I tried setting 
actimeo=0, but that just caused LiveTV to stutter and fail constantly.   
In desperation I switched back to NFSv4 with no special tuning and it 
all just started working properly.

So, thanks for the all the suggestions. Seems like it was my setup.