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RAID Tips 4 of 10 - RAID 5 uncorrectable error probability

By Geek
If you plan on building RAID 5 with a total capacity of more than 10 TB, consider RAID 6 instead. The problem with RAID 5 is that once the member disk had failed, it is required to read the entire array in order to complete the RAID rebuild. Although the probability of encountering a read error in any particular read operation is very low, the chance of a read error occurring increases as the array size increases. It has been widely speculated that the probability of encountering the read error during rebuild becomes practically significant as the array size approaches 10TB. Although the speculation relies on certain assumption which is not likely to be true (we'll have a writeup on that later), consider being better safe than sorry . RAID 6, being capable of correcting two simultaneous read errors, does not have this problem.
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On RAID diagnostic messages

By Geek
Always check what your RAID controller says when a disk failure or other array failure occurs. You should verify by testing that such messages are difficult to miss. Error messages displayed during the boot sequence are no longer useful as the uptime is now measured in weeks even for the home PCs. If the controller doesn't report error messages or for some reason you don't take prompt action to restore the array redundancy once the disk failure has occurred, then there is no point in running a RAID. Using hot spares alleviates the problem for disk failures, but not for a silent controller malfunction. On one of the forums someone told a story about RAID 1 failure where one of the member disks had failed and later it was found out that the second member disk contained the two-month-old data. Two months before the disk failure, the controller lost the array for some reason but didn't report the error. So nobody bothered to restore the redundancy. As a result, when the only re...
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