I recently read a computer science journal about the theory of the function/operation of redundant array of independent disks(RAID). The original acronym included the word inexpensive in lieu of independent.
A RAID is designed to avoid the loss or to enable the recovery of data that may be more valuable than the computers or servers that it is stored on.
A family friend was a data integrity analyst at Yahoo. Yahoo, quite obviously, can not afford to lose a single byte of data. If any of the Yahoo email data was corrupted, a mass exodus of customers and revenue to gmail would certainly ensue. I was quite shocked to learn that many of the Yahoo mail servers have a RAID failure rate of 1.5%.
Now, a 1.5% data loss failure rate may sound impressive, but if one applies this small percentage to the huge number of accounts and resources that Yahoo manages, then such a failure rate is unacceptable.