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RAID
Please read the Partitioning page for background.
RAID comes in a variety of flavors. The most straightforward ones a a RAID mirror, which uses redundant hard drives (duplicates), and RAID striping, which splits the workload across drives.
High performance — a RAID stripe PERMALINK
A RAID stripe uses 2 or more hard drives to store alternating chunks of the data. For example with two drives, even-numbered chunks are stored on one drive, and odd-numbered chunks are stored on the other drive. This splits the workload among the drives. A RAID stripe is said to “scale” if the sustained transfer speed for N drives is N times the speed of a single drive.
High reliability — a RAID mirror PERMALINK
A RAID mirror duplicates data onto two or more disk partitions (on two or more hard drives). A RAID mirror is an exact copy. Delete something, and it’s gone from both.
Keep in mind that human error (eg deleting a file) is instantly reflected in a RAID mirror since all drives in a mirror contain identical copies of the same data. Mirroring is not a backup system, it’s a reliability system. A RAID mirror does not protect you against theft or natural hazards.
If there are N drives (partitions) in the mirror, then N-1 of them can fail with no loss of data. The most common setup is with N = 2 drives. When a drive fails, operation continues seamlessly with the other drive: no data is lost. The failed drive can be replaced, and the mirror can rebuild, automatically (this is one oddball way to backup: swap one of the drives in the mirror with a blank one).
Mirroring generally runs at the speed of a single drive or a bit slower for writes (all drives can be written simultaneously, but there is some overhead). A mirror can offer higher read speed, but not necessarily.
Advanced setups can use both striping and mirroring with multiple partitions on multiple drives. Some advanced RAID configurations require hardware support; Apple sells a RAID card with the Mac Pro.
Backup is still required
A consistent backup strategy is critical, even with mirroring: Acts of Dog can occur (fire, hurricane, lighting bolt, etc). Mirroring is not a backup system, it’s a reliability system.
I don’t use RAID mirror because I value performance, and I backup regularly with both Apple’s Time Machine and with multiple external hard drives in external SATA cases. However, my diglloyd.com web server utilizes a RAID mirror for both the boot volume and the data volume, because no downtime is acceptable.
High performance and reliability— RAID 5 stripe + parity PERMALINK
RAID 5 offers an ideal combination of striping + parity; this allows failure of one drive with no data loss. RAID 5 generally requires hardware support: either a RAID card card that implements it or an external enclosure that offers RAID 5 built-in to the enclosure. See the review of the Other World Computing QX2.
RAID 5 is not as fast as RAID 0 striping, but for many users it will be fast enough.
High performance and more reliability— RAID 10 stripe + mirror PERMALINK
RAID 10 offers striping and mirroring together, to allow for more drive failures. Data is Striped (RAID 0) over mirrored sets (RAID 1) of drives for fast redundancy. This requires four or more drives. For example, with four drives, two mirrors are created of two drives each, and those two mirrors are striped. This allows one drive from each mirror to fail, but failure of both drives in one mirror results in data loss.
RAID 10 requires hardware support: either a RAID card card that implements it or an external enclosure that offers RAID 10 built-in to the enclosure. See the review of the Other World Computing QX2.
RAID 10 is not as fast as RAID 5, but for many users it will be fast enough.
Setting up basic RAID volumes PERMALINK
This section assumes you have two or more hard drives, and explains how to set up a RAID stripe or mirror on them. A RAID stripe offers far better performance than a single drive can (for most tasks). See the Photoshop test results.
A RAID stripe splits the workload across drives, a RAID mirror makes copies on every drive: see Backup and Reliability.
Setting up a basic RAID volume
This example uses two drives. The steps are the same for any number of drives. Apple’s Disk Utility is used for the examples, SoftRAID is also an excellent choice, with a superior user interface.
Note: When you buy a new drive, it’s unformatted, and the disks seen below (Maxtor 7H500F0) might display slightly differently than as shown. Also, if the drives were previously partitioned, you might need to repartition them into single-partitions, use the tab as seen in the example below.
We’re going to create a striped volume named BigBoy consisting of two Maxtor 7H500F0 hard drives (500GB each). A 2-drive striped volume splits data across multiple drives, so the resulting volume size is twice as large for two drives as for one, and twice as fast.
1. Select the tab.
2. Drag each of the hard drives into the area at right (if you have more than two hard drives, drag all the desired drives in)
3. Enter BigBoy for the .
4. For RAID type, choose
5. Click .
Clicking raises a dialog that confirms your choice to erase the drives and make them into a RAID. It’s a good idea to read the dialog and make sure you have selected the desired disks. (I have as many as 12 hard drives attached at any one time, so I do not want to make a mistake!).
After the process completes, you will see a new volume BigBoy appear on your desktop, as well as within the Disk Utility window (see below). Apple’s Disk Utility doesn’t make this very clear, SoftRAID is much better in this regard.
Mirror instead of stripe
Instead of a striped RAID (for high performance), you could have chosen for . A mirrored drive is a good choice when drive failure is not an option, but it is not a substitute for a backup.
In the example below, the volume is named OldReliable, and it shows up as seen below, as well as on the Desktop. Note that a mirror is always the size of the partition on each drive; the data is duplicated across each drive of the mirror. You can have as many duplicates as you wish, and you can add them at any time.
Note also that you can start Disk Utility at any time to check on the mirror status. If one of the drives has failed, you will see an status displayed instead of .
Setting up a two-partition RAID volume PERMALINK
Goal: create a high-performance scratch disk in addition to a large data drive. This is a great solution for Photoshop, especially with 3 or 4 drives. And even though head contention between partitions is in theory an issue, it’s never been one in practice in my usage; it all depends on actual program behavior.
Here we’ll set up two volumes: a high performance scratch volume, and a main data volume with the remaining space. Let’s call these Speedy and Data.
Step 1 — partition the hard drives
First, partition the two drives into two partitions each. Since the final striped volumes will be named Speedy and Data, name the partitions on the first drive Speedy-1 and Data-1, and the partitions on the second drive Speedy-2 and Data-2; this avoids confusion. After this step, there are 4 new volumes that appear on the desktop. If you have 3 or 4 or N drives, partition each in turn into Speed-3/Speedy-4/... and Data-3/Data-4/... etc.
In this example, I want Speedy to be a 64GB scratch volume (striped), so I’ve made Speedy-1 and Speedy-2 be 32GB each. The volumes Data-1 and Data-2 contain the remaining space. Note that four volumes have appeared on the desktop at right.
Step 2 — create the RAID sets
Next, create the two striped RAID volumes, which will each consist of two partitions that you made in step 1.
1. Click on the tab, then drag Speedy-1 and Speedy-2 into the area at right, and make the be Speedy. (The names in the area confusingly change to “disk1s2” and “disk2s2”, that’s life with Disk Utility).
2. Choose , then click .
Repeat this step for Data-1 and Data-2, naming the volume Data.
If you’ve done it all right, you’ll see something like this. The volume icons as seen on the desktop will likely have different icons. If you made a mistake, you can delete the RAID partition and try again.
Conclusions
For more examples, see The DIGLLOYD Mac Pro.
Setting up RAID is not hard, and has many benefits. Users looking for high performance can achieve it cost-effectively, and users who want reliability can have it too. Two drives instead of one drive is all it takes, and drives in 2008 are very inexpensive.
Setting up RAID is even faster and easier with SoftRAID.
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