[mythtv-users] Planning Tuners and Disk I/O for a MythTV Backend System

[Raymond Wagner]

On 2/6/2010 02:46, Jim Beckett wrote:
> SATA1 systems can perform a sustained disk write at ~72 MB/second.
> SATA2 systems can perform a sustained disk write at ~144 MB/second.

No.  SATA1 and 2 are 150MB/s and 300MB/s, respectively. However that is 
the interface bandwidth. It has no bearing on the physical speed of the 
disk. A modern high density disk will average in excess of 100MB/s.
http://www.tomshardware.com/charts/2009-3.5-desktop-hard-drive-charts/h2benchw-3.12-Avg-Write-Throughput,1013.html

> A full transport stream (multiplex, mux) is limited to 38.8 MB/sec 
> data rate.

A full QAM256 multiplex will be ~38mbps, while an ATSC multiplex will be 
~19mbps.  Note the big 'B' and small 'b'.  38mbps means 4.75MB/s.

> That leads me to believe that the practical number of physical tuners 
> for a PVR system can be limited by disk IO throughput, and should be 
> an important consideration when designing, and using the system.

No. You can probably put a dozen digital tuners in a system before you 
might have to worry about exceeding the throughput of a single modern 
hard drive.

> Taking all this into consideration, is there a mechanism in MythTV 
> that recognizes when disk IO limits are be likely to exceeded? If so, 
> does it adjust the recording schedule, or perhaps, show a conflict?

No. You're expected to provide a system capable of the necessary 
throughput for all supplied tuners.  As long as you're not using 
framegrabbers, this should not be a problem.

> If not, then I suppose it would be something that might interest one 
> of the developers. (I'm not up to speed in C/C++) My thought is that 
> it could make the scheduler even smarter if there is a way to 
> determine/predict the likely data recording rate, based on the number 
> of recordings scheduled, and how much system IO resources would be 
> likely to be available at any given time.

MythTV will estimate the storage space a recording will take, and 
auto-expire shows accordingly.  It will also perform load balancing 
across multiple disks if you add them to the storage group as 
independent drives.