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This chapter is from the book

This chapter is from the book

Set Up Software RAID

Before You Begin

Add a Newly Installed Hard Disk to the System

Partition a Hard Disk

Back Up Your Information

For high-performance systems such as Web servers for which disk performance and reliability are paramount, it's important for disks to be available in a redundant and fault-tolerant fashion. This is what RAID (Redundant Array of Independent Devices) provides: a scheme by which you can employ multiple hard disks in a set, mirroring or striping the data from each other, enhancing both access speed and reliability. The downside is that you have to buy at least three times as much disk storage as you need if you don't use RAID technology; but for the kinds of high-reliability applications that RAID is designed for, the cost of hardware is a small concern compared to the need for redundancy.

Figure 3.11Figure 3.11


Key Term - RAID (Redundant Array of Independent Devices)—A technology in which multiple hard disks are harnessed in a set so that data can be stored redundantly (for maximum reliability in case one of the disks goes bad) or across multiple disks at once (to maximize the speed of access).

RAID is often managed by a hardware solution (an add-on card in a high-end system) but this solution is unnecessary with Mac OS X's built-in software RAID option. The procedure for setting up software RAID assumes that you have installed at least two separate disks in the system, in addition to the startup disk. (Yes, that means you need at least three hard drives installed in or attached to your Mac. That's why RAID is intended for high-performance applications only!)

  1. Open Disk Utility

  2. Open the Finder and navigate to the Applications folder, then to the Utilities subfolder. Launch the Disk Utility program by double-clicking its icon.

  3. Open the RAID Pane

  4. First select an entire disk (not a volume) from the left pane of the Disk Utility window. Make sure that you're choosing one of the disks you want to add to the RAID array (and thereby protect its data redundantly or speed up its access with striping), not your Mac's startup disk. Then click the RAID tab to open the RAID setup pane.

  5. Drag Disks to the RAID Set

  6. The left pane of the Disk Utility window lists all the available disks; drag the disks you want to use as the redundant disks in the RAID array into the white RAID set box (do not select the same disk you chose in step 2). You can rearrange the disks into priority order by dragging them in the box, or delete disks from the set by selecting them and clicking the Delete icon above the box. As you add disks to the set, the estimated usable size of the array (depending on your selected RAID scheme) is updated dynamically.


    You must have at least two disks in the RAID set to create a RAID array; neither of these disks can be your startup disk. Creating a RAID array destroys all the data on all the disks you add to the RAID list. Be absolutely sure that you know what you're doing before creating a RAID array!

  7. Select the RAID Scheme

  8. You can select Stripe or Mirror as the RAID scheme. Striping is a good scheme for enhancing access speed because it works by storing parts of the same file across different disks so that all the pieces can be read at once; striping also preserves all your combined usable disk space. However, striping doesn't provide redundancy. Mirroring copies the files across all the RAID volumes, so it does provide redundancy, but not the speed benefits of striping.


    Key Terms - Striping—A RAID data storage scheme in which parts of each file are stored on different disks so that they can all be read independently at the same time for increased access speed.

    Mirroring—A RAID data storage scheme in which all disks in the array are identical copies of each other so that if an error occurs on one disk, the other disks can correct it.

    There are "hybrid" RAID schemes that combine the benefits of both striping and mirroring; however, you must connect at least three RAID drives to your system to use this hybrid RAID scheme, and the scheme isn't supported by Disk Utility (which is, after all, designed for consumers with desktop Macs, rather than the high-availability server applications that use high-level RAID schemes). Professional data-center administrators might want to look into Xserve RAID, Apple's server-class RAID solution.

  9. Choose a RAID Set Name and Format

  10. Enter a descriptive name for the RAID array and choose a volume format. The Mac OS Extended (Journaled) format is the Mac OS X default and is generally the best way to go. You can select Mac OS Extended (Non-Journaled) to obtain a little more speed at the expense of reliability in the case of unexpected shutdowns.

  11. Create the RAID Array

  12. When everything is set up the way you want it, click the Create button. The RAID array is set up and mounted in the Finder as a single new volume, with the name you specified in step 5. You can then use this volume as you would any other disk; all the RAID striping or mirroring takes place under the hood, so you don't have to worry about it any more.

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