Symmetrix DMX RAID 6
207
Data Integrity, Availability, and Protection
Figure 46 on page 208 shows a RAID 6 device consisting of
Symmetrix hypervolumes on eight separate physical drives. Six data
drives (D1–D6) and two parity drives (D7 horizontal parity and D8
diagonal parity).
Figure 47 on page 209 shows a RAID 6 device consisting of
Symmetrix hypervolumes on 16 separate physical drives. Fourteen
data drives (D1–D14) and two parity drives (D15 horizontal parity
and D16 diagonal parity).
Even-Odd algorithm
The algorithm used by Symmetrix for RAID 6 is an XOR based
algorithm called Even-Odd.
This algorithm calculates two types of parity — Horizontal Parity
(HP) and Diagonal Parity (DP), in order to provide protection against
the loss of two members of the RAID group. The two parities are
calculated using different sets of the user data, and are independent
of one another. Horizontal Parity is the equivalent of RAID 5 parity,
and is calculated from the data across all the data disks. Diagonal
Parity (DP) is made up of segments; each DP segment is calculated on
a selected group of data segments. Each DP segment skips a different
data drive in its calculation, which is key to the ability of this
algorithms to reconstruct data after a double drive failure.
Another important requirement of this algorithm is that the number
of data drives used in the DP calculations be a prime number.
Symmetrix offers RAID 6 (6+2) and RAID 6 (14+2) protection (6 data
or 14 data, with 2 parities each), neither of which is 'prime'. The
Symmetrix implementation of Even-Odd assumes there to be 17 data
drives in the RAID group—named D1–D17. For each of the
protection schemes, the 'real' data drives take up positions D1–D6
(for 6+2) or D1–D14 (for 14+2) in the DP calculations; the remaining
drives (Dx up to D17) are 'NULL disks'. These NULL disks exist only
for the sake of the DP calculations, and the 'data' on these NULL
disks is assumed to be all zero when calculating parity. These NULL
disks do not consume any system resources (memory, cache, or
disks). Figure 46 on page 208 and Figure 47 on page 209 show the
RAID 6 data/parity layouts for RAID 6 (6+2) and RAID 6 (14 +2),
respectively.
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