NEC Express5800 series User Manual page 51

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 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
HDDs. If any two of the HDDs 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
Configuring Your Storage and I/O Blade
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)
4-7