Drive failures frequently are correlated and if you absolutely don’t want to lose your data, RAID 6 is the safer of the two options. There’s always the possibility that a second drive will fail during the build of the hot spare process.

Other things to think about are the size of the array. If your RAID array is over a TB then RAID 6 is definitely a better choice because of the long rebuilding process. Also what if an unrecoverable read error occurs while the hot spare is rebuilding, in a RAID 6 environment the second set of parity can be used to continue rebuilding.

What about RAID level 10 to be absolutely sure of safeguarding your data? RAID 10 mirrors everything, therefore an array requires double the disk capacity of the data to be stored. The remainder of the capacity constitutes the mirror.

If a RAID 6 array comprises four disks, only 50% of that space is available as usable capacity, but the proportion of usable space increases as you add more drives. That means half the total capacity of a RAID 10 array will always be dedicated to protection, but with a RAID 6 array the usable capacity grows as the number of drives increases. For example, if you increased the number of disks in a RAID 6 array from four to eight, the space consumed by parity data would decrease from 50% to 25%.

RAID 10 has among the fastest rebuild times possible because it only has to copy from the surviving mirror to rebuild a drive, which can take as little as 30 minutes for drives of approximately 1 TB. The key drawback of RAID 6 is that the time it takes to rebuild the array after a disk failure is lengthy because of the parity calculations required, often up to 24 hours with even a medium-sized array.

RAID 10 doesn’t need special hardware. Most controller hardware will support RAID 10 with good performance. Because RAID 6 doubles the parity calculations for every write, it requires specially designed controller hardware.

Many storage managers have a hard time deciding which redundant array of independent disks level to use for the best availability and reliability. If you are undecided on RAID 1 or 5, consider RAID 1+0 or 6!