RAID 5 Left-Sync: Rotation Formula and Three-Point Verification
Left-sync is a rotation rule, not a guess. Verifying it from data beats trying layouts until something mounts.
The rotation rule
In left-sync, parity starts at the highest member index of the first stripe and moves down one position per stripe, wrapping around. Data fills the remaining positions in ascending order.
Three-point verification
Check three independent points: a filesystem superblock or MFT record that must land at a stripe boundary, the parity XOR result for a full stripe, and the distribution of parity blocks across members. Only a candidate that satisfies all three is accepted.
FAQ
Why not just try all layouts?
Each attempt writes or mounts; wrong assumptions can produce a filesystem that 'works' and silently corrupts files.
Do I need the original controller?
Helpful but not required; the parameters can be derived from the data.