RAID? Was: PATA hard disks, anyone?

[Peter Corlett]

On Wed, Mar 28, 2018 at 05:40:29PM -0700, Richard Pope via cctalk wrote:
> I have been kind of following this thread. I have a question about MTBF. I
> have four HGST UltraStar Enterprise 2TB drives setup in a Hardware RAID 10
> configuration. If the the MTBF is 100,000 Hrs for each drive does this mean
> that the total MTBF is 25,000 Hrs?

That's the mean time before any one disk fails, but not the MTBF for the array
as a whole because failure of an individual disk doesn't cause the array to
fail. There needs to be at least one more disk failure for that to happen.

MTBF is also an overly simple measure which fails to account for the bathtub
curve and correlated failures. Attempts to compute the MTBF of an array from
the MTBF of the individual components will come up with a plausible number
which is technically correct yet bears no relation to the real world.

In practice, the only numbers on a typical hard disk datasheet which aren't
fantasy marketing puff are the physical dimensions and the number of sectors,
and even that is because those are industry-wide standards that disks must
conform to.