RAID-5: Fault-Tolerant vs Failure-Proof
When you link up three or more drives using RAID-5, special parity data is spread across the drives. Thanks to the parity data, if any one hard drive in the array fails, the RAID controller can reconstruct all of the missing data. This is how RAID-5’s fault tolerance works. We elaborate further on the way RAID-5 and its parity data works on our RAID 5 data recovery page.
RAID-5 arrays crash when two or more hard drives fail. Parity data can only reconstruct the contents of one missing hard drive, and so if another hard drive fails, the user’s data becomes filled with holes. In theory, a RAID-5 array should almost never crash like this, because two drives will rarely fail at the exact same time. But unfortunately, that theory lacks a great deal of scientific rigor. The truth is that RAID-5 arrays do crash, and if the frequency of their appearance in our data recovery lab counts for anything, quite often.
Typically, one drive will fail, and when another drive fails, it does so months afterward. RAID-5 users are advised to replace failed drives immediately and begin rebuilding the array as soon as possible. But sometimes, the first drive’s failure goes entirely unnoticed. In other circumstances, the user replaces the drive and begins the process of integrating it into the array. But the rebuilding process puts additional stress on the other drives, which, ironically, can cause another hard drive failure! Sometimes, RAID-5 users will notice a drive failure and put off rebuilding for as long as possible due to this risk—and before they know it, it’s too late.
It’s important not to confuse fault-tolerance with failure-proofing. No data storage devices is perfectly failure-proof. Hence the need to make sure your most important files are backed up.