[mythtv-users] Planning Tuners and Disk I/O for a MythTV Backend System
Raymond Wagner
raymond at wagnerrp.com
Sat Feb 6 08:04:38 UTC 2010
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.
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