HP A7143A RAID160 SA Controller Support Guide, February 2007

RAID Technology Overview

Logical Drives and Arrays

Chapter 1

Figure 1-3 Data Striping (S1-S4) of Data Blocks B1-B12

For data in the logical drive to be readable, the data block sequence must be the same in every stripe. This

sequencing process is performed by the Smart Array Controller, which sends the data blocks to the disk drive,

writing the heads in the correct order.

In a striped array, each physical disk in a logical drive contains the same amount of data. If one physical disk

has a larger capacity than other physical disks in the same logical drive, the extra capacity cannot be used.

A logical drive can extend over more than one channel/connector on the same controller, but it cannot extend

over more than one controller.

Disk failure, although rare, is potentially catastrophic to an array. If a physical disk fails, the logical drive it is

assigned to fails, and all of the data on that logical drive is lost.

Fault Tolerance and Data Protection

To protect against data loss due to physical disk failure, logical drives can be configured with fault tolerance.

The RAID configurations that are supported by the Smart Array Controllers are as follows:

RAID 0 Data Striping only (no fault tolerance)

RAID 1 Data Mirroring only (fault tolerant)

RAID 1+0 Drive Mirroring and Striping (fault tolerant)

RAID 5 Distributed Data Guarding (fault tolerant)

RAID ADG Advanced Data Guarding (fault tolerant)

For any configuration except RAID 0, you can create further protection against data loss by assigning a

physical disk as an online spare (or hot spare). Spare disk drives contain no data and must be in the same

array as the logical drive they are assigned to. Multiple spare disk drives can be assigned to a logical drive,

limited only by the availability of unused physical disks in the array. When a spare disk drive is assigned to a

logical drive, it can only serve as a spare for that logical drive.

When a physical disk in the array fails, the controller automatically rebuilds the information from the failed

disk onto an online spare. The system is quickly restored to full RAID-level data protection. In the unlikely

event that another disk in the array fails while data is being rewritten to the spare, the logical drive may fail,

depending on which RAID configuration is in use. For more information, see Appendix A, “Probability of

Logical Drive Failure,” on page 93.

B11B10 B12