RAID5 capacities / usable drive space ...

[Greg 'groggy' Lehey]

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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