Another feature that Disk Utility and its related command-line tools enable is the use of Redundant Array of Independent Disks (RAID), where multiple physical disks are combined into one virtual disk. In general, RAID solutions are designed for increased data availability and integrity. They trade the overhead of maintaining data across a set of disks for the fault tolerance that using multiple drives can provide.
There are several types of RAID in use today. These are:
- RAID 0
Data is striped across two or more disks giving increased I/O performance; if each disk is on a separate disk controller, this allows for large virtual disks. This type of RAID provides no protection against disk failure. If a disk fails, all the data on the disk set is lost.
- RAID 1
Data is mirrored across two drives, giving complete data redundancy. If one disk fails, the other still has all the data needed to keep going.
- RAID 3, RAID 4, RAID 5
Multiple disks (three or more) are used to both stripe data for performance and to keep parity information to provide for redundancy in case a disk fails. Because data is split and duplicated among many disks, this type of RAID requires significant computation power.
Mac OS X provides support for RAID 0 (striping) and RAID 1 (mirroring). Both types of RAID can be performed with acceptable performance in software. The other forms of RAID are not supported in software, as the increased overhead would create too large a performance hit to be worthwhile. If you require RAID 3, 4, or 5, you should take a close look at hardware solutions such as Apple’s Xserve RAID, which has dedicated processors to handle the extra overhead, or other third-party RAID solutions.
Figure 9-10 shows Disk Utility’s RAID panel. Using this panel, you can perform the majority of tasks involved with using RAID. To create a RAID set using Disk Utility, drag the disks that you want to be part of your RAID to the Disk table, select the RAID scheme, and then click the Create button. If your RAID set is having problems, you may need to rebuild it by clicking the Rebuild button on the RAID panel. Lastly, if you wish to destroy the RAID array, click the Delete Set button.
You can also use the diskutil command to manipulate RAID sets from the command line. For example, to create a RAID set using the command line, use the diskutil createRAID command as follows:
diskutil createRAID [mirror|stripe]setname filesystemtype diskids...
Example 9-6 shows how to create a striped RAID set using two disks with the device IDs of disk2 and disk3.