[mythtv-users] Multiplexing vs multiple cards

[Steven Adeff]

On Thu, Nov 4, 2010 at 1:28 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.
>
> 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?

it's not just recording, but also playback. the more different files
you are reading/writing the more the heads have to move around. I
think you can expect to be able to record a few streams and play back
a few without issue (as long as your database is on a different
drive), but the more you ask the system to do the more your really
going to want to spend the money to add another recording drive.

-- 
Steve
http://www.mythtv.org/wiki/User:Steveadeff
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