[mythtv-users] Building new MythTV system, hardware help

[Blammo]

On 2/22/06, Ask Bjørn Hansen <ask at develooper.com> wrote:
>
> On Feb 22, 2006, at 4:48 PM, James Minihane wrote:
>
> > Here's the question. Would software RAID tax the CPU too much? So much
> > that I would have to bee careful what I do with the MythTV system?
>
> No, it'll be fine.
>
> I actually prefer software raid for the flexibility and better
> integration.

Not the CPU, but the IO bus of the machine. Say, for example you have
a motherboard with Primary IDE, Secondary IDE, and 2 SATA channels,
and let's say a 2 port SATA add on card. You have a (6) drive Raid5
setup,

Primary IDE : 1 boot drive, 1 raid drive
Secondary IDE: cd rom, 1 raid drive
SATA 1 : 1 raid drive
SATA 2: 1 raid drive
PCI SATA 1: 1 raid drive
PCI SATA 2 : 1 raid drive

Given that raid5 basically writes to all drives during a write, and is
checking "sync" between drives/data, with the read/write/verify cycle
of raid 5, during writes, you're going to generate a tremendous amount
of IO across your system buses, between all the points supporting the
drives. All of, which is not only going to affect other things vying
for cpu/IO time, but is going to be affected by them. Fun little
things like "IO Bound on Write" or "Out of Free Buffers" or "choppy
video/audio"...

Trust me, this exact setup is what I tried for 2+ years. I changed
motherboards, processors, ram, cards, drives, kernels, distros, and
could never get it 100% stable. Too many little glitchy things. All of
which went away when I got a HW raid card.

SWRaid can be more flexible, true. SWRaid supports features earlier
(like Raid6) than hardware cards.

SW Raid may work for some people. Heck, with the right design you
could even use it in production, and people do. For myth, I think
you're just asking for trouble to run a multidrive SW raid setup like
the one detailed above.

anyway, just my 2cents.