Although data mirroring, especially remote mirroring, is an important tool in business continuity and disaster recovery, it is not a substitute for conventional backups. While the point has been frequently discussed (see for example the overview section of the Oracle/EMC white paper "No Data Loss Standby Database," which is available at www.emc.com) it isn't always appreciated by managers, users and others.
In this context mirroring refers to making a second complete copy of the data, sometimes as the data is being written ("synchronized mirroring") and often at a remote site ("remote mirroring"). In the event of a disaster or a major failure the enterprise can switch to the synchronized data and continue operating while the fault is being repaired.
Mirroring is an important tool for business continuity since the second copy of the data is available in a matter of seconds or minutes. However it offers no protection against several important causes of data loss, such as accidental deletion or virus infection. Anything that happens to the primary data happens to the secondary data.
While a backup copy's image isn't as up to date as a mirrored copy, that lag makes it possible to recognize system corruption before it affects the backup. In the worst case, the enterprise may have to go to an earlier backup to get rid of the problem.
Rick Cook has been writing about mass storage since the days when the term meant an 80K 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.