When reading technical specification of disks given by vendors it can be noted that often makers provide not real Unrecoverable Error Rate values. This URE probability value is widely utilized to substantiate naive statements similar to "RAID 5 is dead by 2009" and to guess chances of double failure in RAID5. These calculations get people building their own RAIDs concerned.
In fact, the vendor URE data seems to be very far off the mark. Read technical documentation on Hitachi official website, they have kind of interesting URE values for 3 TB hard disk - 10-14 errors per read bit. Such a value can be converted to the probability to read the drive from the start to the end and not encounter an URE is:
(1- 10-14)(8*3*1012)~0,79
therefore, the probability of the disk failing to read one sector is about 20%.
In other words when you have a disk filled at capacity there is a non-negligible chance (about 20%) that you are not be able to get data back off it. This is easily proven wrong by simplistic testing.
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