RAID Technology Overview - September 2007

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 physical disk, 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 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.
Protecting Data With Fault Tolerance and Spare Disks
To protect against data loss due to physical disk failure, logical drives can be configured with
fault tolerance. Fault-tolerant RAID configurations that are supported by the Smart Array
Controllers are as follows:
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 fault-tolerant configuration, you can create further protection against data loss by
assigning a physical disk as an online spare (or “hot spare”). Spare disks contain no data and
must be in the same array as the logical drive they are assigned to. Multiple spare physical disks
can be assigned to a logical drive, limited only by the availability of unused disks in the array.
NOTE: When multiple logical drives are defined on a controller, spare disks must be assigned
to each 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 (page 21).
14 Introduction to RAID Technology