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Disk mirroring is a technique used to protect a computer system from loss of data and other potential losses due to disk failures. In this technique, the data is duplicated by being written to two or more identical hard drives, all of which are connected to one disk controller card. If one hard drive fails, the data can be retrieved from the other mirrored hard drives.
Disk mirroring if often referred to as RAID 1 or RAID Level 1.
Disk mirroring is a form of disk backup in which anything that is written to a disk is simultaneously written to a second disk. This creates fault tolerance in the critical storage systems. If a physical hardware failure occurs in a disk system, the data is not lost, as the other hard disk contains an exact copy of that data.
Mirroring can be either hardware or software based.
Hardware-based mirroring is implemented through the use of RAID controllers installed in the system to which separate hard disk drives are attached. These hard disks appear as different volumes to the system. Each data sector is identically written to all the volumes, thus creating multiple copies of the volumes. At the expense of mild system performance degradation, fault tolerance is introduced to the system.
Software-based mirroring requires certain mirroring applications to be installed in the system. The software-based mirroring solution is usually less expensive and more flexible, but it results in more system performance degradation and is more susceptible to incompatibilities like boot time problems.
A popular alternative to disk mirroring is disk striping, in which data is striped in blocks over multiple volumes (disks). In case of a failure, the failed disk is recreated with the help of checksum or other data present on other disks. Unlike disk mirroring, disk striping may not fully recover lost data.