Re: [Hampshire] Gentoo adaptec RAD controller hangs

On Sat, 8 Nov 2008 15:07:10 +0000 (UTC), nick@??? said:

> > RAID-5 is computationally expensive, and as it
> > relies on at least three drives as opposed to
> > RAID-1's two, it has a shorted MTBF.
>
> Really? Do you have a reference for that?

Suppose, for the sake of example, the disk drives used in the RAID arrays
have an MTBF of one year. In a RAID-1 system, with two drives, you'd
expect two failures per year, or one every six months. With a 3-drive
RAID-5 system, you'd expect three failures a year, or one every four
months. Both RAID-1 and RAID-5 can tolerate the failure of one disk, and
neither can tolerate the failure of a second disk. In the time between one
disk failing and it being replaced and the array rebuilt, you are relying
on two disks remaining operational with RAID-5 and only one with RAID-1.

Of course disks have a much higher MTBF than a year, but the point remains
that in the above scenario, the RAID-1 system has a higher MTBF than the
RAID-5 system.

Keith