[mythtv-users] How many simultaneous recordings on one drive?

[Alex Butcher]

On Wed, 28 Sep 2011, Eric Sharkey wrote:

> On Wed, Sep 28, 2011 at 12:21 PM, Mike Perkins
> <mikep at randomtraveller.org.uk> wrote:
>> Even that might not be enough. Recording four things at once on the same drive
>> means the head is going to be seeking to four different places all the time...
>
> It's not that simple.  File systems don't choose worst-case
> allocations like this in general.
>
> Four recordings at 2 MB/s is 8MB/s.  The buffers on these drives are
> usually 32 or 64MB.  They can seek at almost 100Hz.  It just won't be
> a problem.  You've got a 1996 mind set here.

My experience says the same; I've got a SD DVB-T Mythbox with three tuners,
each providing four virtual tuners using multirec.  That's a total of 12
virtual tuners.  I have two 2TB WD Caviar Black SATA drives (which benchmark
at about 140MB/s at the rim, and about half that at the hub), split into
2x5x200GB RAID1 arrays, and 2x1x200GB arrays for filler recordings.  My SQL
database is in a different RAID1 array, but on the same pair of discs.  UK
DVB-T usually runs at about 1GB/hour (about 0.28MByte/s) per channel or
3Mbyte/s per multiplex, and I've recorded from all 12 virtual tuners without
glitches.

Playback sometimes glitches if a recording starts or finishes whilst
playing (at least, I think that's the cause so far), but that's it. I might
even be able to fix that by playing with using ionice to set realtime
priority for mythbackend/mythfrontend, but I'm reluctant to experiment in
case it causes resource starvation elsewhere.

Obviously HD would be rather more resource-intensive, but not by comparison
with modern hardware.

> Eric

Best Regards,
Alex