Make disk sets in sierra

Learn how to use Disk Util­ity’s RAID As­sis­tant

Mac Format - - CONTENTS - Alan Stone­bridge

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 if 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.

A bonus of up­grad­ing to Sierra is the abil­ity to work with RAID disk sets in Disk Util­ity

A mir­rored set can be re­paired au­to­mat­i­cally or man­u­ally af­ter a disk is dis­con­nected and then re­con­nected.

Take heed of the warn­ing at the fi­nal stage of cre­at­ing a disk set: check you’re happy to erase the disks’ con­tents.

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