RAID5 capacities / usable drive space ...

[Marc G. Fournier]

'K ... I'm going to be setting up a server to test my knowledge here, but,
I've had someone tell me: "the fact that you need a minimum of three
drives in Raid 5, so a three drive configuration in Raid5 is not hot
swappable nor will it boot with less than three working drives." ....

My understanding was that if I had three drives in a RAID5 configuration,
and one died, the file system would still function with the 2 drives ...

Thanks ...

On Wed, 14 May 2003, Greg 'groggy' Lehey wrote:

> On Friday,  9 May 2003 at 22:25:51 -0300, The Hermit Hacker wrote:
> >
> > I have someone telling me something that I'd never heard before, and find
> > difficult to believe ...
> >
> > Apparently, he is under the impression that altho a file system shows a
> > capacity of, say, 100G, its usable space is around 50% of that ...
> > anything higher then that, you risk problems ... (significantly reduced
> > MTBF of the drives, degradation in performance, etc) ...
> >
> > His opinion seems to be based on some talks he had with ppl at IBM and
> > Seagate way back in '89, but still seems to feel they are applicable today
> > ...
> >
> > Is there any fact behind his opinion?
>
> It's difficult to say if he hasn't specified reasons.
>
> I can think of a couple of possibilities.  One would be, of course,
> that RAID-5 always has overhead for parity, and the other is the fact
> that file system performance deteriorates when the file system fills
> up (thus the 10% left over by UFS).  None of these sound like good
> reasons, though.  MTBF depends on the activity, not what kind of data
> (allocated/non-allocated) is on the drives.
>
> Greg
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