[mythtv-users] Frontend for HD Content question.

R. G. Newbury newbury at mandamus.org
Thu Dec 11 16:50:39 UTC 2008

James Crow wrote:
> On Wed, 2008-12-10 at 19:10 -0800, Nick Ellson wrote:
>> Ok, I have tried recording and playback with no setting changes.
>> When I go to Watch TV, Select my first HDHR Tuner and press record and 
>> watch it a second I still see the issues with jitter. I navigate away to 
>> the main memu and wait 10 mins. I come back to Watch Recorded TV and 
>> select the program that is currently recording. It starts off where I 
>> first started recording but the playback is MUCH better. Only tiny 
>> glitches when teh news was showing a football play and there was massive 
>> color changes. It stayed like that while i watched. When I stopped 
>> recording and started playback again, it was the same as the second time.. 
>> Very good playback except for a glitch here and there and it was 
>> consistant glitches so i suspect they got commited to disk as the 
>> recording process was running.
>> Now, last night when it was just nasty, my wife also had two other NTSC 
>> recordings from the PVR500 spooling to disk. So perhaps Disk IO is an 
>> issue. I may need to dedicate a drive for the HD Content... Can I separate 
>> the storage locations per tuner?
>> I also want to look at the idea of dedicating a NIC on the back end just 
>> for the HDHome Run, that sounded like a nice idea. And see how the profile 
>> change works as well.
>> Thanks for all the ideas everyone!
>>   Nick
> Nick, rather than dedicating a drive to HD recordings, just add another
> drive to the default storage group. Let Myth balance the i/o load
> itself. If you have two HD recordings at the same time it should spool
> one to each disk.
> If you really want to have one disk just for HD you can create another
> storage group and set all HD recordings to use it while the SD stuff
> goes to the default storage group.

Nick, you might want to try changing the latency on your drive 
controller. Run 'lspci -vv' and look for 'latency'. Then search the list 
archive at gossamer-threads for instructions on changing the setting.

Also note that some chipsets (Intel for one) seem to always report a 
latency of 0 (zero) but the change appears to work. And I'm not sure 
that the latency has any effect on SATA drives, although it does on IDE.

Regarding storage. It helps to add hard drives to your storage group. 
Myth and the OS will split the write-to-disk into streams so your 
effective throughput goes up and you get less thrashing. If you have 2 
drives and 2 tuners, then each tuner gets to write to a 'dedicated' disk.

It also helps to increase the block allocation size for your video 
drives. This is the space which the driver allocates on the disk, when 
the OS asks for more space (not the same thing as the cluster size, 
although for our purposes, it amounts to the same thing). Since our 
recordings are 2+Gig, an allocation of a half Gig at a time on a 250G or 
500G drive is much more efficient. And it reduces fragmentation greatly, 
increasing the effective write and read speed.
Only available on all file-systems however. I use ext3 on my OS drive, 
but xfs on the 2 video disks. Well worth the time to fix the 'tuning' of 
the drive.


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