Set up a RAID 10, Striped Set of Mirrors, using Mac OS X Disk Utility
The purpose of this series of articles is to provide some documentation on the use of Mac OS X Disk Utility. A previous post demonstrates the need for this documentation increases as more new storage devices show up on the market requiring the use of Disk Utility to implement RAID. The new devices requiring Disk Utility include: the Promise Pegasus J4, the LaCie 5big Thunderbolt, and the USB 3.0 hub RAID, to name a few. This article will cover the creation of RAID 10 a “striped set of mirrors” using Mac OS X Disk Utility. A previous article covered setting up a RAID 0 striped set using Mac OS X Disk Utility..
The reasons for creating a striped RAID set include maximizing the speed and capacity of two or more disks by combining them into a single volume. The reason for creating a RAID 1 mirror is to increase data redundancy thus decreasing the chance of failure because of a disk drive failure. The reason for creating a RAID 10 stripe of mirrors is provide more capacity and speed than a single drive can provide and all of the redundancy of a RAID 1 mirror.
The reasons for not creating a RAID 10 set include reduced capacity and speed from RAID 0 alone and reduced potential redundancy from RAID 1 alone. RAID 10 can be, at most, one half the capacity of a RAID 0 array. Four drives configured in RAID 1 provides 300% redundancy with all four drives being a mirror of each other, while the same 4 drives in RAID 10 provide only 100% redundancy with two mirrored pairs striped together. With a 4 drive RAID 1 you can loose any 3 drives and still retain your data. With a 4 drive RAID 10 you can loose 1 drive from each mirrored set and still retain your data.
Step by step instructions:
Attach all drives to be included in the RAID 10 array to the computer. Raid 10 requires at least 4 drives.
Start Mac OS X Disk Utility. (To find Disk Utility, simply click the magnifying glass in the upper-right corner of the Mac screen and type in “Disk Utility”.)
Ensure all drives to be included in the striped set are visible in Disk Utility.
If any drives were previously part of another raid you may have to “Erase” the drives first.
Select any one of the drives to be included in the striped set and then choose the “RAID” button. Ensure
Format: Mac OS Extended (Journaled)
RAID Type: Mirrored RAID Set
Press the “+” button for each mirrored set you wish to create. For 4 total drives you will press the “+” button twice. For 6 total drives you may press the “+” button 3 times for 3 pair of mirrors, or twice for 2 sets of 3 drives in a mirror.
Name each set appropriately. (i.e.: “Mirror 1, Mirror 2”)
Press the “+” sign once more ensuring:
Format: Mac OS Extended (Journaled)
RAID Type: Striped RAID Set
Drag and drop each drive to their appropriate Mirrored RAID set.
Drag and drop each Mirrored RAID set underneath the Striped RAID set. When done properly each of the Mirrors will we indented from the Striped RAID set.
For each Mirrored RAID set press the “Options” button and select “Automatically rebuild RAID mirror sets”, if desired. If you are not sure, select it.
Press the “Create” button.
When complete, Disk Utility will mount the new drive and the new drive will be visible on your desk top just like any other drive.
In the event a drive becomes disconnected there will be no effect on the system. Work will progress as before. If the drive becomes reconnected the Disk Utility will start a background process that will automatically rebuild the disconnected drive.
In the event a drive fails and must be replaced an automatic rebuild will not occur. To force a rebuild perform the following steps:
Attach the new drive to be included in the RAID set to the computer.
Start Mac OS X Disk Utility.
Select the degraded mirror from the list of drives and RAID sets on the left so that it appears in the window on the right.
Drag and drop the new drive into the degraded mirror.
Press the “Rebuild” button.
For the purposes of this demonstration we replaced 2 drives, one from each mirrored set.
It will be awhile! Do not be alarmed, it is running in the background and should not affect your work.