Using Software Mirroring as a Disk Protection Strategy

Data redundancy is necessary to prevent instances in which a single disk failure can cause a system to go down until the problem is located and corrected. There are two methods of providing data redundancy: software mirroring and hardware mirroring. Each represents RAID Level 1. (See “Using Disk Arrays” for more information on the meaning of the various RAID levels.)

Software mirroring allows you to maintain identical copies of your data (except for the root disk), so that each set of data has, in effect, a perfect clone of itself. In the event a disk fails, the system can use the mirrored copy of the data, thus allowing users to continue to work without interruption. The bad disk can be replaced at a more convenient time when the system can be brought down without causing problems. Once the system is rebooted, the mirroring software will cause the mirrored data to be copied back to the replacement disk and the process of mirroring will begin again.

With three-way disk mirroring, two copies of each disk’s data are maintained. This strategy is even more robust than two-way mirroring which is described above and it eliminates the need to bring the system down at all in order to replace a bad disk.

	NOTE: Using Version 1 LVM Volume Groups, you can have up to three copies of your data (the original plus two mirror copies). Using Version 2 LVM Volume Groups you can have up to six copies of your data (the original plus five mirror copies).

To use disk mirroring, you will need to use LVM or VxVM as your disk management strategy and (if you are using LVM) have available the MirrorDisk/UX software product. MirrorDisk/UX causes every write to the original volume to also be written to the copy or copies of the original volume. Note that the original data and its copied data may be spread over more than one disk.

The main advantage of software mirroring over hardware mirroring, which is discussed in “Using Disk Arrays”, is that the cost of implementation is lower. The main disadvantage of software mirroring relates to its increased complexity of management. That is, it will probably be significantly more difficult to manage a system with a large number of disks as compared to a system with a single disk array.