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RAID-1 Mirror Limitations

Last updated 2011-02-26 - Send Feedback
Related: backup, fault tolerance, RAID, RAID-1

Use of a RAID-1 mirror is appropriate for certain purposes, but is widely misunderstood as to its real benefits and limitations.

A RAID-1 mirror uses two (or more) drives to provide fault tolerance against drive failure, so that when one drive fails, the other drive(s) still have exactly the same stuff. Typically, a mirror uses two drives, so others means one surviving drive.

For example, a mirror with two 2TB drives means there is 4TB of drive storage, but since each 2TB contains the same data, there is 2TB of usable capacity. The user sees one single desktop drive icon having 2TB capacity, not two drives.

Drive failure protection , nothing more

Many (perhaps most) external drive failures are due to the power supply; I hear this over and over about one brand in particular (drive is fine, power supply dies, cheap power supply means a lower retail price).

Every consumer RAID-1 mirror external drive uses a single power supply. So when that power supply fails, the mirror feature is of absolutely no value whatsoever.

Consider—

  • Failure of the power supply renders the mirror feature useless.
  • Failure of the port (USB, Firewire, etc) means the mirror is inaccessible;
  • Theft/fire/flood are equal opportunity destroyers for all drives in a mirror;
  • Impact damage is just as likely to kill both drives in a mirror.
  • A mirrored drive enclosure is a single unit, a single point of failure/loss.
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When to use a RAID-1 mirror

A RAID-1 mirror is for fault tolerance; data remains intact and accessible if one drive fails. It’s imperative that the failed drive is replaced immediately, so that the mirror can be rebuilt, which can take 5-30 hours, depending on drive size. Keep a matching pre-tested spare drive available.

With a 2-drive mirror, you’re then down to one drive, which can fail just like an single drive can. Users with high requirements for data safety should be using a 3-drive RAID-1 mirror, or a 6-8 drive RAID-6, which can also tolerate two failures.

  • Use a mirror for fault tolerance, a mirror is not a substitute for backups!
  • After a drive fails in a 2-drive RAID-1 mirror, you’re just as much at risk as with a single drive, quite possibly more, since whatever caused the failure might affect the other drive (bad batch of drives, power surge, etc).
  • Until a failed drive is replaced and the mirror is rebuilt, you’re at risk. You’ll have to shutdown to replace a drive, and you won’t be protected until the mirror is fully rebuilt, which can take a few hours to a few days, depending on drive size and enclosure technology.
  • Keep a pre-tested and matching spare drive on hand at all times so that it can be popped in as soon as a drive fails.
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RAID-1 mirror for external backup

External backup means making a backup, then storing that backup safely away from the computer.

If the goal of backup is redundancy of the Master data (original copies), then a RAID-1 mirror is definitely not a bad thing, but having twice as many single drives is a superior strategy by far, because greater redundancy in all areas is achieved with separate drives.


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