In data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate
physical hard disks in real time to ensure continuous availability. A mirrored volume is a complete logical representation of separate volume copies. In a Disaster Recovery context, mirroring data over long distance is referred to as storage replication. Depending on the technologies used, replication can be performed synchronously, asynchronously, semi-synchronously, or point-in- time. Replication is enabled via microcode on the disk array controller or via server software. It is typically a proprietary solution, not compatible between various storage vendors.
Mirroring is typically only synchronous. Synchronous writing typically achieves a Recovery Point Objective (RPO) of zero lost data. Asynchronous replication can achieve an RPO of just a few seconds while the remaining methodologies provide an RPO of a few minutes to perhaps several hours. In addition to providing an additional copy of the data for the purpose of redundancy in case of hardware failure, disk mirroring can allow each disk to be accessed separately for reading purposes. Under certain circumstances, this can significantly improve performance as the system can choose for each read which disk can seek most quickly to the required data. This is especially significant where there are several tasks competing for data on the same disk, and thrashing (where the switching between tasks takes up more time than the task itself) can be reduced. This is
an important consideration in hardware configurations that frequently access the data on the disk. In some implementations, the mirrored disk can be split off and used for data backup, allowing the first disk to remain active. However merging the two disks then may require a synchronization period if any write I/O activity has occurred to the mirrored disk.