....and even a 96.05% uptime reguarding hard drive failure over a 5 year MTBF
span, would be unbelievably awesome to an end user. And I would think a
predicted uptime failure rate on hardrives would be taken from a lot of 100
or more hard drives. So that 99% would be more like 99.99999999998765 %
"Chupacabra" wrote:
> "PNutts" <> wrote in message
> news:71E010BD-45B8-4A61-8B60-...
>
> > I'm curious... Why is it considered extra risk? There is a certain risk
> > that
> > any given hard drive will fail, but it remains the same for each drive and
> > is
> > not doubled with two drives. Each drive still carries that same risk. For
> > example, if each drive has a 1 in 100 chance of failure, you have two
> > drives
> > that each have a 1 in 100 chance of failure. If you do combine those
> > numbers,
> > you get a 2 in 200 chance of failure which is still 1 in 100. I hope my
> > examples are clear.
>
> That's not how you calculate cumulative risk though. Running two drives,
> where the failure of either one results in the loss of all data, is slightly
> more risky than running each drive individually.
>
> If the risk of one drive failing is 1%, then its risk of not failing is 99%.
>
> For a two-drive RAID0 set, it would be 99% x 99%, or 98.01%
>
> Instead of a predicted uptime of 99% on a single drive, you only have 98% on
> a two-drive RAID 0 set.
>
> On a 4 drive RAID 0 set, it would be
>
> 99% x 99% x 99% x 99%, or 96.05% predicted uptime.
>
> The more drives you add to a RAID0 set, the lower your predicted uptime will
> be.
>
>
>
>
>
>
>