[mythtv-users] Multiplexing vs multiple cards

[Greg Oliver]

On Thu, Nov 4, 2010 at 12:28 PM, Gavin Hurlbut <gjhurlbu at gmail.com> wrote:
> On Thu, Nov 4, 2010 at 9:35 AM, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
>> As others have said, the demand on your hard drives goes up with every
>> additional stream being recorded. Ideally you would have one drive per
>> possible stream in storage groups. For instance, I have two DVB cards
>> and I have them set to multiplex up to three channels since where I'm
>> at I have a few frequencies with 3 channels. I also have two cable
>> boxes. I have six 500GB SATA drives set up using storage groups. I
>> have yet to have any issues with my hard drives not being able to keep
>> up with the simultaneous recordings, viewings and commflagging.
>
> The demand on the drives obviously goes up with every stream, however,
> I think people are maybe overinflating that demand.  Modern hard
> drives can easily deal with 50-60 MB/s (even the slow ones).  This
> translates to 400-480Mbit/s.  HD streams over the air, etc, use up to
> about 36Mbit/s for the entire multiplex (Is my math right there,
> Raymond?).  Hence, even with "slow" 5400RPM drives, you should have no
> issues at all with the disks if you are being somewhat careful.
> Granted, the seeking overhead kicks in, but even then, even the slow
> drives should be able to easily deal with 7 or 8 *multiplexes*
> recording at a time.  Not many of us even get close to that.

Well, commflagging can occur way faster than the original recording
gets written, so comparing the stream rate to the commflag rate is not
realistic.  One commflag job with high priority done after the fact
can chew my raid array to pieces with IO waits on the cpu.
Efffectively killing the rest of the system.  Obviously reducing the
niceness fixes it, but just making a point.

> Of course, you'd need to keep the mysql database on different drives,
> etc.  But really, if you are using current drives, your drives
> shouldn't be the bottleneck for quite some time.
>
> Am I missing something here?
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>