Make and manage disk sets in macOS

Alan Stonebridge shows how to combine disks for faster transfers or to provide extra protection for files.

Disk Utility in Sierra adds the ability to create and manage disk sets. Strictly speaking, you could also manage disk sets in El Capitan, but you had to do so in Terminal, using the diskutil command. So, an immediate benefit of Sierra is that it’s more comfortable to work with disk sets, whether a prebuilt one that depends on this macOS feature or a set you assemble from separate drives.

A disk set takes multiple disks — a minimum of two — and pools their resources in one of three ways. The first is a striped disk set, or RAID (Redundant Array of Independent Disks) level 0. This type of set has the cumulative capacity of all its disks. It may spread chunks of data, even of part of the same file, across its disks to distribute the burden of read and write operations, improving the set’s overall throughput. However, the risk here is that, if one disk fails, the whole set’s data is lost.

The second is a mirrored disk set, also known as RAID level 1, and for many people, it’s the most compelling of the three options available in Disk Utility due to the additional protection it affords to your files; whatever you copy to a mirrored set is stored multiple times — once on each of the disks in the set. The advantage of this is that, as long as one disk does not fail, you can rebuild the array by replacing those that have become faulty.

The worst cases here are if data on every disk becomes corrupted or all of the disks fail at the same time. The former situation can be addressed by creating one or more periodic backups of the mirrored set and storing that separately, in the hope of limiting propagation of any damage. The latter situation can be dealt with by adding one or more extra disks to the set, either as additional slices (live copies of all the data stored by the set) or as spares, which are ordinarily idle and only called into action when a slice fails, in order to rebuild the set so that you have your desired minimum number of copies.

A mirrored disk set’s capacity is the same as the lowest capacity disk that’s part of it, so you’ll want them to match in order to make the most efficient use of all of them.

The third kind of disk set is JBOD, meaning just a bunch of disks. This simply concatenates the capacity of multiple disks — which can be different — into a single volume. This kind of disk enables several small disks to work like they are one, which you can then use as part of a striped or mirrored array in which other disks match the capacity of the concatenated set. However, this means there are more potential points of failure in the resulting set.

PERFORMANCE CONSIDERATIONS

With a striped disk set in particular, try to connect the disks to different ports on your Mac, rather than through a USB hub, which can detrimentally affect the performance of speedy storage devices, notably SSDs.

In the walkthrough opposite, you’ll learn how to safely set up RAID 0 and RAID 1 disk sets. Before you begin, gather at least two drives that are blank or whose contents you don’t need; they will be erased, so make sure you have a backup of anything on them that you want to keep