[mythtv-users] How many simultaneous recordings on one drive?
mythlist at assursys.co.uk
Wed Sep 28 17:12:58 UTC 2011
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
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.
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