RAIDing your Mac Pro
A concatenated disk set provides no swifter access to your data
By Christopher Breen, Macworld | Techworld | Published: 09:00, 17 May 2007
The Mac Pro and the previous top-of-the-line desktop Macintosh, the Power Mac G5, may look alike on the outside -- cheese-grater exterior, huge brushed-aluminum handles, and USB and FireWire ports up front -- but inside, they're entirely different beasts.
One of the biggest differences (aside from the Mac Pro's Intel Xeon processor) is the number of internal drive slots each Mac has. Off the shelf, the Power Mac G5 can accommodate just two hard drives. The Mac Pro can handle four.
What can you do with the Mac Pro's extra drive bays? One of the coolest things is to fill them with the highest-capacity hard drives you can find and then configure those drives in a RAID (Redundant Array of Independent -- or Inexpensive -- Disks). But what exactly is a RAID, and how do you set one up? Let me explain.
It's a RAID!
As the name implies, a RAID is two or more hard drives configured to share or duplicate data. In other words, instead of storing all your data on just one drive, you store it on several. RAID has been common on servers for years. The Mac Pro makes it a lot easier to implement on the desktop. In a stock Mac Pro, you create a RAID by installing at least one additional drive and then using Apple's Disk Utility to configure the drives.
Whether the RAID shares or duplicates data is determined by its level. Disk Utility supports two RAID levels -- mirrored RAID set (RAID Level 1) and striped RAID set (RAID Level 0) -- as well as something called a concatenated disk set (also known as JBOD, or just a bunch of disks).
At its most basic, a mirrored RAID is composed of two hard drives. Each drive holds the same data; one mirrors the contents of the other. A mirrored RAID has two advantages. It helps protect your data by writing it to two drives rather than to one -- so if one drive fails, you still have all your data on the other. And in some cases, a mirrored RAID can provide better read times.
A striped RAID splits your data among the drives -- parts A, B and C of a file could go on one drive, while parts D, E and F go on another, for example. The advantage of a striped RAID is that it provides better read and write times, particularly if you're working with large files -- video and audio files, for example. The downside of a striped RAID is that because files are spread out among the drives, you lose all your data if one of them goes south.
The other option Disk Utility offers in its RAID pane -- concatenated disk set -- is technically not a RAID.
Rather, it's another way of treating multiple hard disks as a single volume. In this case, the operating system simply combines the storage capacity of all the drives in the set to create what appears to the operating system to be a single volume. A concatenated disk set provides no swifter access to your data, and it offers no additional data protection. But it does allow you to spread large files such as video projects and backup archives across multiple disks.
Prepping the hardware
The Mac Pro accommodates 1-in. Serial ATA 3Gbit/sec. hard drives internally, though you can also use external hard drives to create a RAID. (If you use external drives, be sure to attach each one to its own FireWire bus; FireWire provides greater bandwidth than USB 2.0 in this situation, and daisy-chained FireWire drives make for slower throughput.)
RAIDs work best when you use identical drives of the same capacity. For example, if you created a mirrored RAID with a 300GB drive and a 500GB drive, 200GB of the 500GB drive would be wasted, because the drive would mirror only 300GB. Similarly, you lose any potential advantage of splitting data between drives in a striped array when your smallest-capacity drive is full.
More important, you need to think about how many drives to add. You can, if you wish, use the drive that came with your Mac Pro as part of the RAID. But doing so requires erasing all the data from that drive. If you want to leave your start-up disk alone, you'll need to add at least two new drives to make a RAID.
Whatever kind and number of drives you choose, the Mac Pro makes adding them easy. You just power down and unplug the Mac, and then remove its side panel (by pulling up on the locking latch on its back). Just below the optical-drive carrier at the top of the interior are four slide-out disk-drive carriers (see "Drive Bays"). With the latch still in the up position, pull one of the hard-drive carriers out of its bay. Use the four screws on the drive carrier to fix the hard drive in place, and then slide the carrier back into position. The data connector on the drive will plug into the Mac's drive port automatically. Repeat this procedure for the other drives you're installing (if any), and then close up your Mac -- you're done.
Rigging the RAID
Now comes the fun part: configuring your drives.
If you've installed just one additional drive and are using the hard drive that came with your Mac Pro as part of the RAID, you'll need to boot from your OS X Install disc; then you set up the RAID with the disc's copy of Disk Utility.
In the process, you'll have to erase all the data on your start-up drive.
Insert the first Install disc, and then double-click on Install Mac OS X And Bundled Software in the Installer window. In the resulting Install Mac OS X window, click on Restart, and enter your administrator password when prompted. When the Mac boots up, choose your language in the window that appears, and click on the Next arrow. When the Installer application launches, choose Utilities: Disk Utility.
If you aren't using the Mac Pro's original start-up drive as part of your RAID array, you can simply install two (or more) new drives and then launch the copy of Disk Utility that's on your start-up drive (/Applications/ Disk Utility).
Inside Disk Utility
You use Disk Utility to configure your disks as a striped RAID, a mirrored RAID or a concatenated disk set.
To begin, select a drive that you want to add to your RAID (or disk set) from the list of drives on the left side of the Disk Utility window. Click on the RAID tab in Disk Utility's main window, name the RAID in the RAID Set Name field, and choose the type from the RAID Type pop-up menu. Leave the Volume Format menu set to Mac OS Extended (Journaled).
Drag the disks that you want to be part of the RAID into the list box in Disk Utility's main window (see "Adding Disks"). The RAID Set Estimated Size total will reflect the amount of storage your RAID will have. If you've chosen a striped RAID or a concatenated disk set, the amount of total storage will be the sum of the drives' capacities. If you've chosen a mirrored RAID, the total will be the capacity of the smallest drive in the RAID (since the configuration will duplicate data across the drives).
You now have a couple of configuration options. If you're creating a mirrored RAID, you can determine whether a disk will be an active part of the RAID (known as a slice) or a backup (known as a spare). If used as a slice, the drive will store the RAID's data at all times. If designated as a spare, the drive will mirror data only when another drive fails.
Obviously, if you have a RAID that's made up of only two drives, you'll have no spare -- each drive must be an active member of the RAID. But if you have a third drive, it could wait in the wings in case one of the other drives suddenly goes kablooey. You make this choice by selecting a drive you've added to Disk Utility's main window and then selecting either RAID Slice or Spare from the RAID Type pop-up menu.
You also have the option of setting the RAID's block size -- from 16KB to 256KB.
(The block size determines the size of the chunks of data that are written to the drive.) Click on Disk Utility's Options button and choose a block size from the RAID Block Size pop-up menu (see "Choose Block Size"). Smaller block sizes are the way to go if your RAID will be primarily devoted to reading and writing small files such as text and database files. If you deal with larger chunks of data -- beefy video and audio files, for example -- choosing a block size of 256KB will move your data more efficiently.
In the same pane is the RAID Mirror AutoRebuild option. When this option is enabled, the data in a mirrored RAID will be automatically rebuilt if one of the drives fails and a spare needs to take over, or if you've disconnected a drive from the RAID and it needs to be updated when you reconnect it. This rebuild happens in the background.
Striped RAIDs and concatenated sets require almost no configuration -- you pretty much set them and go.
When your RAID is configured to your satisfaction, click on Create. Doing so will erase the data on all drives in the RAID. (However, when you add a drive to an existing concatenated disk set, only the data on that new drive will be erased; the data on the existing disk set remains. Once the new disk has been erased, its capacity will be added to the disk set.)
You can, if you wish, combine multiple RAIDs. For example, you can configure two drives to act as one striped RAID, configure another two drives to act as a second striped RAID, and then combine those two striped RAIDs in a single mirrored RAID. Such a setup combines the speed advantages of a striped RAID with the safety of a mirrored RAID.
To do this, create the two striped RAIDs, according to the preceding instructions. (You can save a bit of time by dragging two drives into Disk Utility's main window, configuring the first set, clicking on the plus-sign [+] button at the bottom of the window to create a second set, and then dragging the two other drives into it.)
With the two sets created, drag each of them to Disk Utility's main window, where the program will treat them just like hard drives (see "Multiple RAIDs"). Choose the RAID type you want, and click on Create to create the RAID.
When you're done, your RAID will appear as just another volume on your Mac, and it can be treated accordingly; you can install applications, play movies and create documents on it -- in short, you can do exactly what you'd do with any other hard drive attached to your Mac.
Senior Editor Christopher Breen is the author of The iPod and iTunes Pocket Guide, Second Edition (Peachpit Press, 2007).