RAID Technology Overview - September 2007

The advantages of RAID 1+0 are as follows:
• Highest read and write performance of any fault-tolerant configuration.
• No loss of data as long as no of failed disks are mirrored to any other failed disk (up to half
of the physical disks in the array can fail).
The disadvantages of RAID 1+0 are as follows:
• Expensive — many disks are needed for fault tolerance.
• Only 50% of total disk capacity usable for data storage.
RAID 5: Distributed Data Guarding
RAID 5 uses a parity data formula to create fault tolerance. In RAID 5, one block in each data
stripe contains parity data that is calculated for the other data blocks in that stripe. The blocks
of parity data are distributed over the physical disks that make up the logical drive, with each
physical disk containing only one block of parity data (see Figure 2-4). When a physical disk
fails, the data that was on the failed disk can be calculated from the parity data in the data blocks
on the remaining physical disks in the logical drive. This recovered data is usually written to an
online spare in a process called a rebuild.
RAID 5 is useful when cost, performance, and data availability are all equally important.
Figure 2-4 Distributed Data Guarding, Showing Parity Information (Px,y)
The advantages of RAID 5 are as follows:
• High read performance
• No loss of data if one physical disk fails.
• More usable disk capacity than with RAID 1+0; parity information only requires the storage
space equivalent to one physical disk on the array.
The disadvantages of RAID 5 are as follows:
• Relatively low write performance
• Data loss occurs if a second disk fails before data from the first failed disk is rebuilt.
RAID ADG: Advanced Data Guarding
RAID Advanced Data Guarding (ADG), sometimes referred to as RAID 6, is similar to RAID 5
in that parity data is generated and stored to protect against data loss caused by physical disk
failure. However, with RAID ADG two different sets of parity data are generated for each data
block on a stripe. The two parity data blocks are stored on different physical disks, allowing data
to be preserved even if two physical disks fail simultaneously. Figure 2-5 illustrates the two sets
of parity data that require as much storage capacity as the data blocks they correspond to on
each stripe in a logical drive.
RAID 5: Distributed Data Guarding 17