Summary

Redundancy is one of the guiding principles for storage networks. Of the three forms of redundancyduplication, parity, and deltaduplication as instantiated in mirroring and backup products has been the most heavily implemented over the course of several decades.

Mirroring is a simple concept developed for online, real-time duplication of storage I/O commands to two targets in a mirrored pair. The location of the mirroring operator can have a significant impact on the overall availability of a system and its data. It is often implemented at multiple levels to provide protection from different kinds of threats.

Often mirroring is confused with two other related and complementary redundancy applications: multipathing and remote copy. It's important to keep the differences between these technologies clear to be able to build and manage complete redundancy solutions.

The essence of mirroring is to access two storage targets, both of them online and operating in real-time to achieve high system availability. The essence of multipathing is to achieve system availability by providing different paths that access a single storage target. The essence of remote copy is to establish a remote storage target for high data availability and is intended to be accessed by another system in a remote location.