[mythtv-users] Newbie

[Simon Hobson]

Gavin Whitehead wrote:

>So are there whole threads dedicated to which are the best disks to use
>[sound of can-of-worms opening] ?

Oh dear, not another annelid inundation :-/

Yes, there have been discussions, no I don't recall much of a 
consensus being reached. The only consensus I think we'll reach at 
the moment is that now is not a good time to be buying disk drives.


Eric Sharkey wrote:

>What are you doing that could possibly exceed the seek rate for a
>mythTV recordings disk?  The mysql database is often seek bound, but
>streaming  a half dozen or so recordings?  That doesn't make any sense
>unless you're trying to run your backend with 16MB of RAM or have a
>seriously fragmented filesystem.

Amount of memory has little effect while recording - though it can 
help greatly with real-time concurrent commflagging or transcoding by 
allowing that process to use the recording data that's been cached in 
memory instead o fhaving to read it again (thus adding to the disk 
seek activity).

Myth will sync the recording file every second. So for each recording 
that's active, there will be at least 2 seeks/second - seek, write 
data, seek, write filesystem data, and repeat ... I suspect on many 
filesystems there may well be 3 seeks as the OS writes the journal as 
well. That's a minimum, for some writes, there may be multiple 
operations updating the filesystem data (such as when a new extent 
needs to be added to the file.)

It's done this way as, without this, the OS would cache large amounts 
of data in RAM, and then go to write it out in a big chunk. This can 
tie up the system to the extent that buffers start overrunning, and 
your recording gets corrupted.


I have been considering fiddling with the parameters, but TBH I'm 
unlikely to find the time. I'm wondering if a larger buffer and less 
frequent syncs might improve overall performance.

And don't forget that doing other tasks (eg transcoding or 
commflagging) will also impact on performance.

I haven't done any tests to find the limits on my current system, but 
on my previous system it could only cope with 2 recordings while 
watching another, and without commflagging. That was a Xen guest 
without a dedicated storage disk though.

-- 
Simon Hobson

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