What Is RAID 10, and Does My Mac Support It?

RAID 10 Definition and Considerations for Implementing It on Your Mac

RAID 10 illustration
RAID 10 is a nested RAID array involving a set of striped mirrors.

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RAID 10 is a nested RAID system created by combining RAID 1 and RAID 0. The combination is known as a stripe of mirrors. In this arrangement, data is striped much as it is in a RAID 0 array. The difference is that each member of the striped set has its data mirrored. This ensures that if any single drive in the RAID 10 array fails, the data isn't lost.

One way to think of a RAID 10 array is as a RAID 0 with an online backup of each RAID element ready to go, should a drive fail.

RAID 10 Requirements

RAID 10 requires a minimum of four drives and can be expanded in pairs; you can have a RAID 10 array with 4, 6, 8, 10, or more drives. RAID 10 should be composed of equal-size drives.

Advantages and Disadvantages

RAID 10 benefits from very fast read performance. Writing to the array can be slightly slower because multiple write locations on the array members must be found. Even with the write being slower, RAID 10 doesn't suffer from the very low speeds seen in random read and writes of RAID levels that use parity, such as RAID 3 or RAID 5.

You don't get the random read/write performance for free, however. RAID 10 requires more drives; four as a minimum vs. three for RAID 3 and RAID 5. In addition, RAID 3 and RAID 5 can be expanded one disk at a time, while RAID 10 requires two disks.

RAID 10 is a good choice for general data storage, including serving as a startup drive, and as storage for large files, such as multimedia.

Calculating an Array's Size

A RAID 10 array's size can be calculated by multiplying a single drive's storage size by half the number of drives in the array:

S = d * (1/2 n)

"S" is the size of the RAID 10 array, "d" is the storage size of the smallest single drive, and "n" is the number of drives in the array.

RAID 10 and Your Mac

RAID 10 is a supported RAID level available in Disk Utility up to OS X Yosemite. With the release of OS X El Capitan, Apple removed direct support for all RAID levels from within Disk Utility, but you can still create and manage RAID arrays in El Capitan and later using Terminal and the appleRAID command.

Creating a RAID 10 array in Disk Utility requires you to first create two pairs of RAID 1 (Mirror) arrays, and then use them as the two volumes to be combined into a RAID 0 (Striped) array.

Bandwidth Considerations

One issue with RAID 10 and a Mac that is often overlooked is the amount of bandwidth needed to support the software-based RAID system used by OS X. Beyond the overhead of having OS X manage the RAID array, there's also the need for a minimum of four high-performance I/O channels to connect the drives to your Mac.

Common ways to make the connection are to use USB 3, Thunderbolt, or in the case of 2012 and earlier Mac Pros, the internal drive bays. The issue is that in the case of USB 3, most Macs don't have four independent USB ports; instead, they're often connected to one or two USB 3 controllers, thus forcing multiple USB ports to share the resources available from a controller chip. This can limit the potential performance of software-based RAID 10 on most Macs.

While it has a great deal more bandwidth available, Thunderbolt can still have the problem of how many Thunderbolt ports on your Mac are controlled independently. In the case of the 2013 Mac Pro, there are six Thunderbolt ports, but only three Thunderbolt controllers, each controller handling the data throughput for two Thunderbolt ports. MacBook Airs, MacBook Pros, Mac minis, and iMacs all have a single Thunderbolt controller shared with two Thunderbolt ports. The exception is the smaller MacBook Air, which has a single Thunderbolt port.

Overcoming Bandwidth Limitations

One method of overcoming the bandwidth limitations caused by shared USB or Thunderbolt controllers is to use a pair of hardware-based RAID 1 (Mirrored) external enclosures, and then use Disk Utility to stripe the pair of mirrors, creating a RAID 10 array that only needs two independent USB ports or a single Thunderbolt port (due to the higher bandwidth available).

Also Known As

RAID 1 + 0, RAID 1 & 0

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