RAID, or Redundant Arrays of Independent Disks, is a technology that enables two or more hard disks to act as one, allowing you to achieve data redundancy or greater effective disk speed. Types of RAID are named for "levels" 0 through 5. The numbers represent different combinations of redundancy, speed, and other attributes.
Disk Utility used with Mac OS X 10.1 through 10.3.9 offers RAID levels 0 and 1. A RAID set of two or more hard disks may be level 0 or 1, but not both, when set up with Disk Utility. Third-party products may offer different levels of RAID.
Disk Utility used with Mac OS X 10.4 or later offers RAID levels 0, 1, and RAID concatenation. Concatenation allows you to combine different RAID types to combine their benefits. For example you can combine RAID 1 and RAID 0 to create a RAID 10.
RAID 0 (striping) for improved disk speed
Striping improves speed by exploiting the computer's ability to deliver data to a hard disk faster than it can write it. Data is written to two or more drives alternatively, instead of just one, improving overall system speed. This approach provides performance improvements, but it does not provide data redundancy. If the hard disks are not the same capacity, the smaller size will be the amount of disk space used on both drives. Disk Utility determines this automatically. A striped array appears as one logical volume in the Finder.
RAID 1 (mirroring) for disk redundancy
Mirroring improves reliability by writing the same data to two or more disks. If one hard drive mechanism should fail, the second disk still contains all the data. At least two hard drives are required for mirroring. Mirroring does not provide performance improvements. If the hard disks are not the same capacity, the smaller size will be the amount of disk space used on both drives. Disk Utility determines this automatically.
Important: RAID volumes created with Mac OS 10.1 cannot be used by Mac OS 9.
Steps to set up RAID
Setting up a RAID array in Mac OS X is part of the installation process. This procedure assumes that you have already installed Mac OS 10.1 and the hard drive subsystem (two hard drives and a PCI controller card, for example) that RAID will be implemented on. Follow these steps:
- 1. Open Disk Utility (/Applications/Utilities).
2. When the disks appear in the pane on the left, select the disks you wish to be in the array and drag them to the disk panel.
3. Choose Stripe or Mirror from the RAID Scheme pop-up menu.
4. Name the RAID set.
5. Choose a volume format. The size of the array will be automatically determined based on what you selected.
6. Click Create.
Recovering from a hard drive failure on a mirrored array
- 1. Open Disk Utility in (/Applications/Utilities).
2. Click the RAID tab. If an issue has occurred, a dialog box will appear that describes it.
3. If an issue with the disk is indicated, click Rebuild.
4. If Rebuild does not work, shut down the computer and replace the damaged hard disk.
5. Repeat steps 1 and 2.
6. Drag the icon of the new disk on top of that of the removed disk.
7. Click Rebuild.