NEC Express5800 User Manual page 112

4-42 Configuring Your Server
RAID5
In RAID5, data is distributed to hard disk drives by striping and, at the same time, the parity
(redundant data) is distributed to the hard disk drives. 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 hard disk drive. Accordingly, the total capacity assigned to the parity is just the same as the
capacity of a single hard disk drive. If any one of the hard disk drives configuring a logical drive is
defected, 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
hard disk drives. If any two of the hard disk drives configuring a logical drive are defected, data is
still available with no problems.
HDD 1
Stripe 1
Stripe 4
Parity P (5,6)
Parity Q (7,8)
HDD 1
Stripe 1
Stripe 4
Parity (5, 6)
HDD 2
Stripe 2
Parity P (3,4)
Parity Q (5,6)
Stripe 7
RAID Controller
HDD 2
Stripe 2
Parity (3, 4)
Stripe 5
RAID Controller
HDD 3
Parity P (1,2)
Parity Q (3,4)
Stripe 5
Stripe 8
HDD 3
Parity (1, 2)
Stripe 3
Stripe 6
HDD 4
Parity Q (1,2)
Stripe 3
Stripe 6
Parity P (7,8)