The Mechanism
RAID 50 divides its total pool of disks into distinct RAID 5 groups. For example, a 12-drive RAID 50 array might be split into:
- Sub-Array 1 (RAID 5): Drives 1, 2, 3, 4
- Sub-Array 2 (RAID 5): Drives 5, 6, 7, 8
- Sub-Array 3 (RAID 5): Drives 9, 10, 11, 12
The top layer then stripes data (RAID 0) across Sub-Arrays 1, 2, and 3. Because each individual sub-array is running RAID 5, each group can only survive one drive failure.
The Impact on RAID 50
If Drive 1 and Drive 5 fail at the same time, the array survives completely fine because they belong to different sub-arrays. However, if Drive 1 and Drive 2 (both inside Sub-Array 1) fail simultaneously, Sub-Array 1 collapses. Because the top-level RAID 0 layer requires every single sub-array to be functional to read the overall stripe, the loss of that single RAID 5 "leg" instantly destroys the entire RAID 50 volume.
