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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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