[mythtv-users] Where to dedicate processing power?

[Steven Adeff]

On 3/7/06, Anthony Vito <anthony.vito at gmail.com> wrote:
> > Setup a RAID 5 array for recordings (since you'll have 4 tuners total you'll
> > want the speed),
>
> This is a common misconception around here. A high bandwidth mpeg2
> stream in SD is only 8 M(bits)ps or about .8 M(bytes)ps 4 of them are
> a mezzly 3.2 M(bytes)ps. Most modern harddrives can sustain 60
> M(bytes)ps. So there isn't much of an issue here. It 'can' become a
> problem when a few tuners are writing and a few tuners are reading. (

but with the possibility of 4 recording, 2 viewing and commercial
flagging you can easily go beyond a single harddrive's abilitiy to
perform read and write opperations.

> I would like to find some numbers on how many writing tuners and
> reading frontends it takes to max out a single disk. )

this all depends on bitrate of the recordings. a single harddisk can
only perform one operation at a time, and it can't switch between
reading and writing very fast, so it can do all write operations or
all read operations quickly, but switching back and forth slows both
down. While writing 4 streams for recording shouldn't be difficult for
SD video, throw a read operation or two in and now your slowing things
down a lot. Considering you can buy 250GB hdd's for <$100, buy three,
have tons of recording space and don't worry about speed.

> I Assume you
> didn't mean a hardware RAID 5 solution, and RAID 5 in software will
> give about about the same performance as RAID 1 on the read, and much
> lower performance on the write.

no, software raid should be fine, he's only got a P3-933, but even on
that md shouldn't take up too much CPU. Considering it won't be used
as a frontend I think he'll do ok. buy more ram...


> Back to the OP. Since this lad doesn't have much CPU power to begin
> with, I'd suggest a single large disk. If you wish for redundency and
> couldn't bear to lose recordings in a crash, or if the single disk
> does become a bottle neck, get another identical disk and build a RAID
> 1. This can be done non-destructively and at any point in the future.

I'm going to say go with a 3 drive software RAID5 with some 250GB
drives. You can have a spare drive or use all three (though you won't
get immediate redundency without a spare, but you will get more space
and can add a spare later, but you can't easily add another storage
drive). Obviously if you don't want to get new drives, go ahead and
begin with your largest drive for recordings and see how it works out
for you.

> Also, I agree with a previous suggestion to keep the Tuner cards all
> in one place, and in the same machine as the drives. This will further
> reduce CPU usage and network traffic.

It will also allow you to only require one machine to be always on to
never miss a recording. But make sure you have a good powersupply in
this machine!!

--
Steve