Chapter 2. RAID Array Controller

2.6.4 RAID 4

RAID 4 (Figure 2–8) breaks up host data into chunks, calculates parity by per- forming an exclusive-or on the chunks, and then writes the chunks to all but one drive in the array and the parity data to the last drive. When the host requests data from the disk drives, the controller retrieves the chunks containing the ad- dressed data, reconstitutes the data from the chunks, and passes the data to the host.

Figure 2–8 Diagram of a RAID 4 Write

Host Data

Controller divides the

 

data into chunksized units

 

0011

0110

and calculates parity

 

 

 

 

 

 

1011

0000

 

0011

0110

 

Striped data and parity

0110

1111

 

1011

0000

 

 

 

written to the array

1110

1110

 

0110

1111

 

 

 

 

 

 

XOr

1110

1110

 

 

 

 

Parity =

 

 

 

 

 

 

0000

 

 

 

 

 

 

 

 

 

 

There is still data left

so the Controller

repeats the Process

 

0011

0110

Striped data and parity

 

1011

0000

 

written to the array

 

0110

1111

 

 

XOr

1110

1110

 

Parity =

 

 

 

 

0111

 

SHR-1057

EK–SMCPQ–UG. C01

2–13

Page 46
Image 46
Compaq 3000 manual Diagram of a RAID 4 Write