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Using RAID 5 sub-arrays
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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.
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.
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
P (40-43) P (44-47)C40 C41 C42 C43 C44 C45 Stripe 5
C36 C37 Stripe 4
C27 C28 C30
C38
Stripe 3
C19 C21 P (20-23) Stripe 2
C12 P (12-15) C13 Stripe 1
C3 P (4-7) C4 C5 Stripe 0
P (32-35)C33 C34 C35
C29C25 C26 P (24-27)
C20C17 P (16-19) C18
C11P (8-11) C9 C10
C2C0 C1
C46
P (28-31)
C22
C14
C6
C39
C47
C31
C23
C15
C7
P (36-39)C32
C24
C16
C8
P (0-3)
PPC60 C61 C62 C63 C64 C65
C52 C53
C39 C40 C42
C54
C27 C29 P
C16 P C17
C3 P C4 C5
PC49 C50 C51
C41C37 C38 P
C28C25 P C26
C15PC13C14
C2C0 C1
C66
P
C30
C18
C6
C55
C67
C43
C31
C19
C7
P
P C68
C56
C45
C57
C32 C33
C20 P
PC8
C44
C69
C46
P
C21
C9
C58
C70
P
C34
C22
C10
C59
Stripe 5
Stripe 4
Stripe 3
Stripe 2
Stripe 1
Stripe 0
C71
C47
C35
C23
C11
PC48
C36
C24
C12
P