Gateway 840 VT-100 manual 236

Using RAID 5 sub-arrays

it is still only necessary to cluster four write commands together, as opposed to the seven necessary as indicated below. The array of drives appears as two separate sub-arrays, each with its own rotating parity.

P (40-43)	C40	C41	C42	C43	P (44-47)	C44	C45	C46	C47	Stripe 5
C32	C33	C34	C35	P (32-35)	C36	C37	C38	C39	P (36-39)	Stripe 4
C24	C25	C26	P (24-27)	C27	C28	C29	C30	P (28-31)	C31	Stripe 3
C16	C17	P (16-19)	C18	C19	C20	C21	P (20-23)	C22	C23	Stripe 2
C8	P (8-11)	C9	C10	C11	C12	P (12-15)	C13	C14	C15	Stripe 1
P (0-3)	C0	C1	C2	C3	P (4-7)	C4	C5	C6	C7	Stripe 0

It can be seen that the more sub-arrays used, the more likely it is for a Full Stripe Write to occur. We recommend that you use either four or five drives in a sub-array, for best performance. The following table shows that even with 15 drives, it is still possible to perform Full Stripe Writes, by clustering together 4 chunks of data.

P

C60

C61

C62

C63

P

C64

C65

C66

C67

P

C68

C69

C70

C71

Stripe 5

C48

C49

C50

C51

P

C52

C53

C54

C55

P

C56

C57

C58

C59

P

Stripe 4

C36

C37

C38

P

C39

C40

C41

C42

P

C43

C44

C45

C46

P

C47

Stripe 3

C24

C25

P

C26

C27

C28

C29

P

C30

C31

C32

C33

P

C34

C35

Stripe 2

C12

P

C13

C14

C15

C16

P

C17

C18

C19

C20

P

C21

C22

C23

Stripe 1

P

C0

C1

C2

C3

P

C4

C5

C6

C7

P

C8

C9

C10

C11

Stripe 0

As well as the performance advantage gained from using multiple sub-arrays, there are a number of other advantages.

Experiencing multiple drive failures

In a configuration with multiple sub-arrays, it is possible for the array to sustain multiple drive failures, provided that there is only one failure in each sub-array.

Promoting faster rebuild

A rebuild operation must read data and calculate parity from all the remaining drives in the array. If multiple sub-arrays are used, this means that it is only necessary to read the data from the remaining drives in the sub-array, not all

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