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RAID 1 Mirroring

A RAID mirror* replicates data onto two or more disk partitions (onto two or more hard drives); this is a form of fault tolerance.

When a drive fails, operation continues seamlessly with the other drive: no data is lost. The failed drive can be replaced, and the mirror can rebuild, automatically (this is one oddball way to backup: swap one of the drives in the mirror with a blank one).

RAID-1 mirror
2 or more drives,
one volume, one icon

A RAID mirror is a single volume that looks and behaves exactly like a single “drive” (volume): one icon on the desktop. That is the beauty of a mirror: there is absolutely no difference in usage as compared to a single drive/volume, yet failure of one drive maintains all data and functionality.

Typically a RAID-1 mirror consists of the entire capacity of all the drives in the mirror set but it is possible to mirror partitions across drives.

* The term “mirror” is historical and apparently chosen by someone who never looked into one (hint: try to read printed matter in a mirror). A better term would have been “RAID Replica”.

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2 or more mirrors

If there are N drives (partitions) in the mirror, then N-1 of them can fail with no loss of data. The most common setup is with N = 2 drives.

A RAID-1 mirror can be created in software with two or more independent drives, or via hardware such as a dual-drive unit like the OWC Mercury Elite Pro Dual.

Mirroring generally runs at the speed of a single drive or a bit slower for writes (all drives can be written simultaneously, but there is some overhead). A mirror can offer higher read speed, but not necessarily.

Backup is still required

Mirroring is not a backup system, it’s a fault tolerance system. For example, a RAID mirror does not protect against theft or natural hazards.

A consistent backup strategy is critical, even with mirroring: Acts of Dog can occur (fire, hurricane, lighting bolt, etc).

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Creating a software RAID-1 mirror

See the RAID-1 how-to page.

Creating a RAID-1 mirror with DiskUtility
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