The topics included in this chapter are as follows:
• RAID 1E (Data Mirroring and Striping)
• RAID 5 (Striping with Parity)
• RAID 6 (Striping with Dual Parity)
• RAID 10 (Mirroring and Striping)
• RAID 50 (Striping RAID 5 Arrays)
• RAID 60 (Striping RAID 6 Arrays)
Redundant Array of Independent Disks (RAID) is a storage technology consisting of one or more disks working in parallel. RAID technology allows an array of physical disks to be combined together to form a virtual disk, which appears as a single storage device to the user.
You can configure a RAID virtual disk in several different ways, with each configuration offering its own benefits. This chapter provides information about the properties of each available RAID level and how to choose a suitable RAID level.
RAID 0 stripes data across multiple disks. However, it is important to remember that data is divided between multiple physical disks, so data is not recoverable if one of the physical disks fails. RAID 0 is ideally suited for applications requiring best performance but fault-tolerance is not a concern.
RAID 0 is known as striping because data stripes size on the virtual disk are divided among a number of physical disks, as shown in Figure 1-1. Consequently, the speed at which data is read and written is increased because data stored on multiple physical disks can be read and written simultaneously. However, it is important to remember that data is divided between multiple physical disks, so data is not recoverable if one of the physical disks fails.
RAID 1 performs disk-mirroring. RAID 1 can increase Read performance. Data is identical on both of the physical disks, so a read command can be sent to the physical disk that is not busy, rather than wait for the primary physical disk to finish an in-process read. However, RAID 1 does incur a slight drop in Write performance because both physical disks must be written to when sending data to the virtual disk. See Figure 1-2 for more details.
RAID 1 also incorporates fault-tolerance, which allows access to the data even if one physical disk fails. In addition, if you replace the failed physical disk with a new physical disk, the rebuild begins recreating the mirror set of drives, which can be accomplished while the system continues to function normally.
A.4 RAID 1E (Data Mirroring and Striping)
RAID 1E is a combination of data mirroring and striping (RAID 0) techniques. At least three physical disks are needed to configure a RAID 1E virtual disk. Data is striped and mirrored across the disk array. RAID 1E is similar to RAID 10, except that RAID 1E operates on an odd number of disks. See Figure 1-3 for more details.
RAID 1E provides the read and write performance of RAID 0, while providing the redundancy protection of RAID 1.
Figure 1-3 RAID 1E: Mirroring and Striping
Note: Like RAID 1, RAID 1E uses the mirroring technique for redundancy. Three 80 GB physical disks (a total of 240 GB), for example, would only store 120 GB of original data because the remaining 120 GB is required to mirror the original data.
A.5 RAID 5 (Striping with Parity)
RAID 5 uses the striping technique in combination with parity to provide high fault-tolerance. At least three physical disks are needed to configure a RAID 5 virtual disk, as shown in Figure 1-4. Data and parity information are striped across all physical disks, with the RAID parity information requiring the equivalent of one physical disk, regardless of the number of physical disks. Consequently, when three physical disks are used in a RAID 5, one-third of usable capacity is lost. The capacity efficiency increases as the total number of physical devices increases.
The combination of striping and parity provides high fault-tolerance with a more efficient use of physical disk space than RAID 1 (mirroring). Read performance is very good but there are overheads for write operations.
Figure 1-4 RAID 5: Striping with Parity
A.6 RAID 6 (Striping with Dual Parity)
RAID 6 uses the striping technique in combination with dual parity to provide high fault-tolerance. At least four physical disks are needed to configure a RAID 6 virtual disk, as shown in Figure 1-5. Data information is striped across all physical disks and parity information is stored twice on different physical disks. The RAID parity information requires two physical disks, regardless of the number of virtual disks. Consequently, when four physical disks are used in a RAID 5, half of the usable capacity is lost; however, the percentage of lost capacity reduces as the total number of physical disks increases.
The combination of striping and parity provides high fault-tolerance with a more efficient use of physical disk space than RAID 1 (mirroring) if the array is comprised of more than four physical disks. Read performance is also very good, but write performance is slower because of the need to calculate and write parity data twice.
RAID 6 is similar to RAID 5, but dual parity means that data can be recovered when up to two disks fail.
Figure 1-5 RAID 6: Striping with Dual Parity
A.7 RAID 10 (Mirroring and Striping)
RAID 10 incorporates the technique of striping (RAID 0) across multiple mirrors (RAID 1). At least four physical disks are needed to configure a RAID 10 virtual disk. See Figure 1-6 for more details.
RAID 10 provides the read and write performance of RAID 0, while providing the redundancy protection of RAID 1.
Figure 1-6 RAID 10: Striping and Mirroring
Note: All mirror-technique RAID levels use 50% of the capacity for fault tolerance.
A.8 RAID 50 (Striping RAID 5 Arrays)
RAID 50 incorporates the technique of striping (RAID 0) across multiple RAID 5 arrays. The RAID 5 arrays use the striping technique in combination with parity to provide high fault-tolerance. At least six physical disks are needed to configure a RAID 50 virtual disk, as shown in Figure 1-7.
Figure 1-7 RAID 50: Striping RAID 5 Arrays
A.9 RAID 60 (Striping RAID 6 Arrays)
RAID 60 incorporates the technique of striping (RAID 0) across multiple RAID 6 arrays. RAID 6 uses the striping technique in combination with dual parity to provide high fault-tolerance. At least eight physical disks are needed to configure a RAID 60 virtual disk, as shown in Figure 1-8.
Figure 1-8 RAID 60: Striping RAID 6 Arrays
Each RAID level has features that are more suitable for some applications than they are for others. So consider carefully which RAID level is best suited for your application.
Table 1-1 gives an overview of the features for each RAID level.
Features |
RAID 0 |
RAID 1 |
RAID 1E |
RAID 5 |
RAID 6 |
RAID 10 |
RAID 50 |
RAID 60 |
Read Performance |
Best |
Poor |
Good |
Good |
Good |
Good |
Good |
Good |
Write Performance |
Best |
Poor |
Good |
Good |
Poor |
Good |
Good |
Poor |
Fault Tolerance |
Poor |
Best |
Good |
Good |
Best |
Good |
Good |
Best |
Efficient use of disk capacity |
Best |
Poor |
Poor |
Good |
Good |
Poor |
Good |
Good |
Automatic rebuild |
No |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Minimum number of drives |
2 |
2 |
3 |
3 |
4 |
4 |
6 |
8 |