7.5. RAID 1-Disk Mirroring

7.5. RAID 1 Disk Mirroring

RAID 1 is used in mission-critical environments where performance and fault tolerance are a requirement for smaller data sets, such as system disks, root master files, and database journals.

The implementation of RAID 1 is shown in Figure 7-5.

Figure 7-5. RAID 1 implementation.

7.5.1 Disk Mirroring

Disk mirroring uses two disk drives of identical size. Data is written to one drive and an exact copy is written on the second disk. If one drive fails, the mirrored drive ensures data is not lost and read-write operations continue to be served. As a result of the duplication of all data, available disk space is 50% of raw capacity.

All HP Smart Array controllers support hardware-based mirroring. Software-based mirroring does not require drives of identical size, but it does require operating system support. Data is written to partitions of equal size.

7.5.2 Disk Duplexing

Disk duplexing works like mirroring. However, disk duplexing uses two disk controllers rather than one, which increases fault tolerance. The logical drive is available even in the case of a controller failure. As a result of the duplication of all data, available disk space is 50% of raw capacity.

Note

Duplexing must be handled by the operating system. (This is also true for duplexed HP Smart Array controllers.)

Disk mirroring and disk duplexing are illustrated in Figure 7-6.

Figure 7-6. Disk mirroring and disk duplexing.

7.5.3 RAID 1 Performance

In a multidisk configuration, RAID 1 mirrors each pair of disks to each other. These disk pairs can then be striped to create a virtual disk. RAID 1 can tolerate multiple disk failures if no two disks forming a mirrored pair fail. Some RAID implementations support n-way mirroring, where the original data is duplicated n times on n disks.

The performance in read environments is high, especially when the array controller takes advantage of split seeks (choosing one disk over the other based on lower seek times). RAID 1 incurs a slight write penalty caused by duplication of the write requests.

Note

The HP implementation of drive mirroring is accomplished with hardware. Drive mirroring also can be implemented in software at the operating system level. Software mirroring adds additional overhead to the processor and is often less efficient than hardware mirroring.

The limitation of RAID 1 is that it is an expensive solution because it requires twice as much drive storage to store duplicate data. Only 50% of the total disk space is available for data storage.