HP D5970A - NetServer - LCII Configuration Manual page 19

Chapter 2
RAID 5: Striping with Distributed Parity
RAID 5 is the most common configuration because it provides good overall
performance and data protection with a minimum loss of storage capacity.
RAID 5 distributes the parity blocks equally among all disk drives to achieve
better overall performance than if a dedicated parity disk is used (RAID 3). If you
have five physical drives configured as one RAID 5 logical drive, data blocks are
written as follows:
Disk 1
Stripe 1
Block 1
Stripe 2 Block 5
Stripe 3 Block 9
RAID 5 outperforms RAID 1 for read operations. The write performance,
however, may be slower than RAID 1, especially if most writes are small and
random. For example, to change Block 1 in the diagram above, the integrated HP
NetRAID controller must first read Blocks 2, 3, and 4 before it can calculate
Parity Block 1-4. Once it has calculated the new Parity Block 1-4, it must write
Block 1 and Parity Block 1-4.
RAID 5 Advantages
There is no data loss or system interruption due to disk failure, because if one
disk fails, data can be rebuilt.
Capacity equivalent to only one disk in the RAID 5 logical drive is reserved to
store redundant data.
RAID 5 outperforms RAID 1 for read operations.
RAID 5 gives good performance if you have a high volume of small, random
transfers.
RAID 5 Disadvantages
Write performance is slower than RAID 0 or RAID 1.
RAID 5 Summary
Choose RAID 5 if cost, availability, and performance are equally important.
RAID 5 performs best if you have I/O-intensive, high read/write ratio
applications such as transaction processing.
Disk 2 Disk 3
Block 2 Block 3
Block 6 Block 7
Block 10
Parity 9-12
RAID Overview
Disk 4 Disk 5
Block 4 Parity 1-4
Block 8
Parity 5-8
Block 11 Block 12
13