Peter, > No, you're confusing RAID-3 and RAID-4/5. In RAID-3, sequential blocks > are organized as: > > DISKS ------------------------------------> > 0 1 2 3 PARITY > 4 5 6 7 PARITY > 8 9 10 11 PARITY > 12 13 14 15 PARITY > > ... whereas in RAID-4 with a chunk size of four blocks it's: > > DISKS ------------------------------------> > 0 4 8 12 PARITY > 1 5 9 13 PARITY > 2 6 10 14 PARITY > 3 7 11 15 PARITY The description you have for RAID-3 is wrong. What you give as RAID-3 is actually RAID-4 with a 1 block segment size. RAID-3 uses BITWISE (or BYTEWISE) striping with parity, as opposed to the BLOCKWISE striping with parity in RAID-4/5. In RAID-3, every I/O transaction (regardless of size) accesses all drives in the array (a read doesn't have to access the parity). This gives high transfer rates, even on single-block transactions. DISKS ------------------------------------> 0.0 0.1 0.2 0.3 PARITY 1.0 1.1 1.2 1.3 PARITY 2.0 2.1 2.2 2.3 PARITY 3.0 3.1 3.2 3.3 PARITY It is NOT possible to read just 0.0 from the array. If you were to read the raw physical device, the results would be meaningless. The structure of the array limits the number of spindles to one more than a power of two (3, 5, 9, 17, etc.). I have seen this implemented with 5 and 9 drives (actually, the 9 was done with parallel heads). Peter Ashford - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
