A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful when read performance or reliability are more important than data storage capacity. Such an array can only be as big as the smallest member disk. A classic RAID 1 mirrored pair contains two disks (see diagram), which increases reliability geometrically over a single disk. Since each member contains a complete copy of the data, and can be addressed independently, ordinary wear-and-tear reliability is raised by the power of the number of self-contained copies.
To understand RAID 1 failure rates consider a RAID 1 with two identical models of a disk drive with a weekly probability of failure of 1:500. Assuming defective drives are replaced weekly, the installation would carry a 1:250,000 probability of failure for a given week. That is, the likelihood that the RAID array is down due to mechanical failure during any given week is the product of the likelihoods of failure of both drives. In other words, if the probability of failure is 1 in 500 and if the failures are statistically independent then the probability of both drives failing is
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