CertiGuide to A+ (A+ 4 Real)  9  Chapter 10: The Small Computer System Interface, SCSI       9  RAID            9  RAID Levels

Duplexing 1 RAID-0/1

RAID-5¹²⁵ provides independent access via striping with distributed parity, as in Figure 406. RAID-5 is a good choice for file and server applications. It is often used for Internet and intranet servers because it provides a good balance between performance, fault-tolerance and cost.

Parity information must be updated when data is written to a RAID-5 array. This is actually a 6-step process:

1. Reading the old data.
   
   
2. Writing the new data.
   
   
3. XOR the old data with the new data.
   
   
4. Read the old parity from the array.
   
   
5. XOR the new data parity with the old data parity.
   
   
6. Write the updated parity to the array.

Every write request to a RAID-5 array must perform two reads, two writes and two XOR operations to complete the original write request. RAID-5 write operations are 33% to 66% slower than RAID-1 operations because of the parity generation. The overall number of drives in the RAID-5 array also af-fects performance. When a drive fails in the array, the remaining drives must read the XOR parity and reconstruct the missing data. Read and write operations can be significantly affected when operating the array in a de-graded mode.

Duplexing 1 RAID-0/1