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Old June 14th 18, 09:39 AM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
Chris
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Posts: 832
Default External hard drive advice please

Wolf K wrote:
On 2018-06-13 14:56, mechanic wrote:
On Tue, 12 Jun 2018 20:21:53 -0400, Wolf K wrote:

Probability of failure of all three at once will be 1/9th of
probability of failure of any one of them, which IMO is low
enough. :-)


Eh?


If I remember probability theory correctly, then if each device has the
same probability of failure, then the probability that two will fail at
the same time is (1/2)^2. If there are three, it will be (1/3)^2. And so
on. This is the reason that a RAID system is more reliable than any of
the drives in it.

If I've misremembered the probability math, kindly correct it (and save
me the work of checking it myself. :-) )


It's not quite right. The probability of three independent disks all
failing at the same is the cube of the individual probability: P(A)^3

If the individual probability is (not at all realistic) 0.1 (10%), then the
probability of all three is 0.001 (0.1%).

RAID is a bit complicated as the probability of a failure is highly
correlated. Firstly, the potential risk failure is if *any* of the disks
fail, which is the sum of the probabilities which for a 5 disk array is 5 x
0.1 = 0.5 (50%). Again not realistic.

Probability increases with age of the disk, so a 5 year old drive is more
likely to fail than a brand new one. This is where correlated failures
occur, particularly with RAID5, as when a disk fails the array needs to be
rebuilt putting a large strain on the existing (likely old) disks which can
cause another one of them to fail. The array is now dead and unrecoverable
which is one reason why RAID5 is not recommended.

In terms of raw disk failure probabilities RAID arrays are no more reliable
than separate disks, however, the redundancy and checksums allow for
seamless recovery from failures.

This is why a RAID is not a backup.




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