When -- and when NOT -- to use mirroring

Mirroring provides a failsafe for data, but it's not without its pitfalls. Here's when and when not to use the features of mirroring.

What you will learn from this tip: Mirroring provides a failsafe for data, but it's not without its pitfalls. Here's when and when not to use the features of mirroring.


Mirroring is a powerful data protection option that provides redundancy and fast recovery. While the detailed process of establishing mirrors depends on the operating system and storage management software used, there are some general principles that make mirroring easier.

1. Weigh your options.

The major drawback to mirroring, of is storage capacity. At the very least a true mirror doubles storage needed for a given amount of data. There are other techniques, such as snapshots or on-the-fly delta, that can provide access to a damaged or missing file nearly as fast as mirroring, without the high storage overhead.

Often, your storage management software will provide these options in addition to mirroring.

2. Know that mirroring is not backup.

Don't rely on mirrors as your only form of data protection. Mirrors are very good for quickly recovering lost or damaged files, but are no substitute for backup.

3. Mirror to more than one physical disk.

With the prevalence of desktop systems with single large hard drives, there is a temptation to mirror LUNs to the same physical disk. This provides only very limited protection and should be avoided no matter how much money it saves.

4. Use identical disks for your mirror.

The smallest disk in the mirrored set establishes the size of the mirror. For performance reasons, it is best to match the speed and other characteristics of the disks as well. Ideally, you should use disks of the same make and model to establish capacity.

5. Have multiple paths of access.

Disks aren't the only things that can fail. If you're going to spend the money for mirrored storage, you should also consider how you'll protect access from other possible failures, such as the controller or network. For anything above a desktop or a very small server, you should seriously consider providing a redundant access to your mirrored data. Having more than one path to your mirror is good insurance and usually inexpensive to boot.

For more information:

Advice: Software mirroring vs. hardware mirroring

Tip: The critical link in mirroring

Advice: Mirroring data from a file server to an external disk


About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
This was first published in February 2005

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