Appendix A: Optimizing RAID 5 Write Performance

Introduction

With a typical RAID 5 implementation, there are a number of steps necessary to write the data to the media. Every write from the host system will typically generate two XOR operations and their associated data transfers to two drives. If the accesses are sequential, the parity information will be updated a number of times in succession. However, if the host writes sufficient data to cover a complete stripe, the parity data does not need to be updated for each write. It can be recalculated instead. This operation takes only one XOR operation per host write, compared to two for a standard RAID 5 write. The number of data transfers necessary are also reduced, increasing the available bandwidth. This type of write access is termed a Full Stripe Write.

P (20-23)

C20

C21

C22

C23

Stripe 5

 

 

 

 

 

 

C16

C17

C18

C19

P (16-19)

Stripe 4

 

 

 

 

 

 

C12

C13

C14

P (12-15)

C15

Stripe 3

 

 

 

 

 

 

C8

C9

P (8-11)

C10

C11

Stripe 2

 

 

 

 

 

 

C4

P (4-7)

C5

C6

C7

Stripe 1

 

 

 

 

 

 

P (0-3)

C0

C1

C2

C3

Stripe 0

 

 

 

 

 

 

The table shows the distribution of data chunks (denoted by Cx) and their associated parity (denoted by P(y-z)) in a RAID 5 array of five drives. An array is defined as a set of drives, on which data is distributed. An array will have one RAID level. A chunk is the amount of contiguous data stored on one drive before the controller switches over to the next drive. This parameter is adjustable from 64 K to 256 K, and should be carefully chosen to match the access sizes of the operating system. A stripe is a set of disk chunks in an array with the same address. In the above example, Stripe 0 consists of C0, C1, C2, and C3 and their associated parity.

Maximum performance will be achieved when all drives are performing multiple commands in parallel. To take advantage of a Full Stripe Write, the host has to send enough data to the controller. This can be accomplished in two ways. If the host sends one command with sufficient data to fill a stripe, then the controller can perform a Full Stripe Write. Alternatively, if the host sends multiple sequential commands, smaller than a stripe size (typically matching the chunk size), the controller can internally combine these commands to get the same effect. In the above example, if a 256 K chunk size

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