[vdr] Re: "stand-by-recording"

[Emil Naepflein <Emil.Naepflein@philosys.de>]

On Wed, 6 Nov 2002 14:01:03 +0100 (CET), _cooper_ <cooper@linvdr.org>
wrote:

> You're talking about writing 24 hours a day, 7 days a week on a hard disk
> without any pause. Not only that it would rise the power consumption, desktop
> hard drives are just not designed for continuous writing, in most cases
> they're not even specified to run 24/7. Only server hard disks (yes, they are
> some expensive, even with IDE) are specified for this.

Are you doing live viewing 24/7 with your VDR machine? ;-)

Ok, you probably don't powerdown your VDR when not in use. But this can
be fixed by an inactivity timeout or a "soft powerdown" to stop
background recording when no one is viewing.

> Probably we will have then hard disk failures earlier, so a lower MTBF.

In our company PCs with normal IDE drivers run since many years 24/7
without any failures.

My own VDR server which is regularily doing more than 6 h recording per
day and for more than 5 hours viewing a day runs now about 2 years
without any problems. Anyway, for me a disk failure wouldn't be so
tragic because I am running 7 80 GB Maxtors in Raid-5 configuration. ;-)

> I'll talk tomorrow to a technician of Western Digital to point out, how bad a
> continously writing to a desktop hard drive really is and what we'll have to
> expect. I'll write the summary here to the list.
> Just to avoid the extra heating, power consumption (even a harddisk can
> power down), additional noise and not to overstress the harddisk I would
> really think of buying more RAM, let's say a Gig or more, for having 30
> minutes recording buffer. Even with 512 MB you have room for approx. 15
> minutes, with 256 MByte it's about 5 Minutes (leaving 64 MB free for the Linux
> system).

64 MB is not enough if you do regular recording, too. At least with mv 4
DVB-S system and software raid I need 256 MB to run smoothly under full
load.

The real question is what should we expect from this feature.
If you just want to jump back a few seconds or even minutes, the ram
buffer/disk approach is fine. But if you want to make the whole
transmission permanent after you have viewed it and it was really nice,
then you are talking about several gigabytes buffer.

What are the settop boxes doing which have this feature. I can remember
at least one which has a buffer for half an hour. I don't think that
they have enough ram to keep the data. The key is to limit the
background recording to the time where you are doing live viewing. Then
the load on the disk is not very high. Remember, the data rate is only
about 500 KB/s. This means you have to write every 30s only for about
one second. This is nothing for a disk. Please think also about the
pause feature. If you do this also only to ram then you are very limited
on the time you can pause.

For me personally this feature has not much use, because since the VDR
server is running we practically do not much live viewing anymore. Most
interesting transmissions are programmed through tvtv.de for nearly a
month in advance. A daily look into the program adds some more
recordings. I have regularily more than 100 active timers (beware the
loadvdr.pl script of epg2timers must be fixed to handle more than 100
timers). All is viewed timeshifted to jump over the commercials. So
going back a to any point of the recording is no problem. To be useful
for jumping over commercials the buffering must be at least 30 minutes
to get enough headroom. This would mean at least 1 GB more ram. For this
money I would rather buy another 200 GB disk space. ;-) 

Anyway, the implementer of this features will finally decide how to do
it.  ;-)

Cheers,

Emil


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