
38 High Performance Parallel I/O
The interconnect between the controllers and the drive drawers is a pair of
redundant 3-Gbps Serial Attached SCSI (SAS) links. ALCF uses DDNs 8+2P
data protection scheme which implements RAID 3 (byte striping) with 2 par-
ity bytes.
1
The parity groups are distributed vertically through the drawers.
If you consider the array to be a 3D matrix of hard drives, with the x and
y dimensions the rows and columns inside the drawer, and the z dimension
going vertically up and down the drawers, then the 10 hard drives that are
in the same x and y coordinates in each drawer form a parity group. This ar-
rangement means that two entire ...