# Re: backup archive format saved to disk

```Andrew Sackville-West wrote:
```
```On Wed, Dec 06, 2006 at 02:52:29PM +0100, Johannes Wiedersich wrote:

```
```Question: how likely is it that both disks develop bad blocks, while
none of them is damaged? I'm no expert on this, but I guess a better
strategy might be to rotate backups on two disks, and use (and check:
fsck and smartctl) them reguarly.
```
```

if the chance of a disk failure is (say) 1% in the time alloted, then
the chance of having a failure with disks is 2%. THe change of any one
```
```
I don't follow this reasoning. Are you presuming independence of the
failures and identical probabilities? If so, then this is the way to
compute it:

Let p be the probability of failure of each disc, independently of the
other. There are four mutually independent events which comprise the
space. Both discs may fail [Pr = p^2]. The first disc may fail, while
the second does not [Pr = p(1-p)]. The second disc may fail, while the
first does not [Pr = (1-p)p]. Both discs may survive [Pr = (1-p)(1-p)].

So, the probability that at least one disc fails is 1-(1-p)(1-p).
For p = 0.01, that is 0.0199.

I'll grant you this is not markedly different from 2%, but it is also
not simply 2p.

```
```particular disk failing is still 1%, it the odds of A failure in the
system as a whole that goes up. So with more disks you're more likely
to have failures of some kind, but the per disk failure stays the same
and the odds of losing ALL of them goes the other way. The odds of
losing BOTH disks is .1%. the question becomes, which one has
failed...
```
```
I don't follow this reasoning. The probability of both discs failing
(if they do so independently) is not 0.1%, but rather 0.01%. A partially
failed disc is usually easy to detect, since they have FEC on them. A
completely failed disc is even easier to detect :-)

Mike
--
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```