**fact**

When I was first told this, my initial response was “*rubbish*“, there was no way that using RAID1 (mirroring) could increase the chance of disk failure, was there?

Then I actually thought about the statement again, and **I was wrong**!

Consider 2 hard disks **A** and **B**. The make/model/manufacturer is irrelevant for this *proof*.

Given that **P(e)** is the *probability of event ***e** occurring, we can state that:

P(disk **A** fails) = **a**

P(disk **B** fails) = **b**

Trivially, in a single disk system using either disk **A** or disk **B**, the probability of disk failure will be **a** or **b**, or for ease of demonstration we shall pick the average of the two.

Therefore, the average chance of a single disk machine suffering disk failure is **(a+b)/2**.

Now, consider the RAID1 system containing disks **A** and **B**.

The chance of a disk failing is the probability of disk **A** failing *or* the probability of disk **B** failing; so:

P(disk **A** fails or disk **B** fails) = P(disk **A** fails) + P(disk **B** fails) = **a** + **b**.

In fact, the chance of disk failure in a RAID1 system *doubles*. Hardly surprising if you stop and think about it.

Now, the statement I initially inferred actually was: *RAID1 increases chances of data loss*, which is obviously rubbish, as can be easily shown.

We know the chance of disk failure (and in this case *data loss*) with a single disk, **(a+b)/2**.

Now, using RAID1, the chance of data loss is defined as:

*The probability of both disk ***A** and disk **B** failing.

This is defined as:

P(disk **A** failing and disk **B** failing) = P(disk **A** failing) * P(disk **B** failing) = **a * b**.

It can be stated that any probability must be in the range 0 <= P(**e**) <= 1.

For any two numbers **i**,**j**; if both **i** and **j** satisfy 0 <= **i** <= 1; 0 <= **j** <= 1 then:

**i * j < i**; **i * j < j**

Therefore, the probability of *data loss* is lower when using RAID1, however the chance of *disk failure* doubles!