Professional Hardwarenear Programming
Storage Systems
© 2012 Peter Thoemmes, 2012-12-15
RAID Systems using Big Physical Disks
A serious problem that had came up when disk size grew is the fading reliability of RAID systems. The bigger a physical disk is, the more likely you will experience an unrecoverable bit error when reading it. So it comes that RAID1 (mirror) and RAID5 show up as a useless effort when building up a 4TB size virtual disk using physical disks with an URE (unrecoverable error rate) of 10^14. Rebuilding a 4TB mirror (two 4TB disks) from degraded mode (1 of 2 disks failed) will fail with a probability of about 35%. The same is true for a 4TB RAID5 (three 2TB disks) which rebuilds from degraded mode (1 of 3 disks failed). Only rebuilding a 4TB RAID6 (four 2TB disks) which was degraded by just one disk (1 of 4 disks failed) will fail with an acceptable probability of about 10^-10. Here you can download the full calculation and an interesting approach with a 12TB SW-RAID6 on top of five 4TB HW-RAID6 (each made of four 2TB disks plus 2 dedicated hot-spares) which fails its rebuild from degraded mode with a probability of only 10^-39 (best case) to 10^-9 (worst case), depending of the health status of the five virtual HW-RAID6 disks. So you'll get approximately the same safety for a 12TB virtual SW-RAID6 disk as for a 4TB virtual HW-RAID6 disk, using that setup on top of your HW-RAID setup.