linux at thehobsons.co.uk
Sat Dec 3 18:02:20 UTC 2011
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
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.
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