Creating a RAID in OS X 10.11

With the release of OS X 10.11 El Capitan, Apple has removed the ability to create RAIDs using the Disk Utility application. Despite the loss of this feature, users still have the ability to create RAIDs on systems running OS X 10.11 by using the Terminal application and some useful commands.


Disk Utility Commands

Although the familiar Disk Utility application has been somewhat simplified in 10.11, a full-featured Disk Utility is still available through Terminal's command line interface. 

OS X can create the following software RAID types:

stripe (RAID 0) A volume that spans two or more devices
mirror (RAID 1) Where data is written to one volume and duplicated on a second
concat A concatenated volume spanning multiple drives that can be of different sizes


Examining the man pages for diskutil we find several commands that allow users to mimic many of the features found in previous versions of the Disk Utility application. Since the focus of this blog post is on RAIDs, the following commands are applicable:

diskutil listRAID Display Apple RAID volumes with current status and associated member disks
diskutil createRAID [mirror, stripe, concat] Create an Apple RAID set in the specified RAID type


Creating a RAID

In this scenario a new striped RAID will be created using two 2 GB USB drives. The drives are formatted, using Disk Utility, to OS X Extended (Journaled) and named "Disk1" and "Disk2."


Formatting of USB drives in Disk Utility (OS X 10.11)


Once these drives have been formatted, the Terminal application is opened.

First we must determine what the disk number is for each of the USB drives. To accomplish this we use the command diskutil list.


Terminal running diskutil list command (note USB drives are disk2 and disk3)


In order to create the RAID we need to call diskutil and tell it to create a striped RAID, name the RAID, provide a file system format, and assign disks to the RAID. Here's a breakdown of the command to show what each part indicates.


With our two disks attached to the system as /dev/disk2 and /dev/disk3, the RAID is created using the command outlined above.


Terminal application showing RAID creation


Viewing Information About the RAID

Once the RAID is created, it is automatically mounted and available to be used in OS X.


Disk Utility showing existence of RAID


Viewing the new RAID in Disk Utility reveals some information, but if more extensive details are needed, the RAID can be examined in Terminal. Running thediskutil listRAID command scans the system for attached RAIDs and provides valuable information on the RAID and its members.


Terminal showing results of the command diskutil listRAID


Examining the results of this command, we can determine the following:

Name Name of the RAID (special device) given upon creation
Unique ID The UUID of the RAID set
Type Type of RAID as defined during creation (stripe, mirror, concatenated)
Status The status of the RAID (Online = available, Offline = unavailable)
Device Node The disk entry given to the device when attached to the computer


Below this information we see extended information about the individual members making up the RAID. Each disk entry is shown, along with the status of the disk and its relevant size. It is important to note that each disk entry will receive its own unique device identifier (UUID), and these assist in rebuilding the RAID if required.

Need more? Contact BlackBag Technologies' training team for more information, or to register for a seat in one of our upcoming classes.

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