NEC Express5800 120Rj-2 User Manual page 88

User Guide
RAID5
In RAID5, data is distributed to HDDs by striping and, at the same time, the parity
(redundant data) is distributed to the HDDs. This mode is called "striping with
distributed parity".
Each of stripe x, stripe x+1, and parity (x, x+1) created from stripe x and stripe x+1 is
written onto a specific HDD. Accordingly, the total capacity assigned to the parity is
just the same as the capacity of a single HDD. If any one of the HDDs configuring a
virtual drive is defective, data is still available with no problems.
RAID6
A RAID 6 extends RAID 5 by adding an additional parity block (Q) created by
different calculation method such as weighting by some factor, thus it uses block-level
striping with two parity blocks distributed across all member disks. This mode is called
"striping with duplex and distributed parity". Accordingly, the total capacity assigned
to the parity is just the same as the capacity of two HDDs. If any two of the HDDs
configuring a virtual drive are defective, data is still available with no problems.
Parity P (5,6)
Parity Q (7,8)
RAID Controller
HDD 1
Stripe 1
Stripe 4
Parity (5, 6)
Figure 34: RAID5 Configuration
RAID Controller
HDD 1 HDD 2
Stripe 1 Stripe 2
Stripe 4 Parity P (3,4)
Parity Q (5,6)
Stripe 7
Figure 35: RAID6 Configuration
HDD 2 HDD 3
Stripe 2
Parity (1, 2)
Parity (3, 4)
Stripe 3
Stripe 5
Stripe 6
HDD 3
Parity P (1,2)
Parity Q (3,4)
Stripe 5
Stripe 8
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HDD 4
Parity Q (1,2)
Stripe 3
Stripe 6
Parity P (7,8)