In
data storageData storage can refer to:* Computer data storage; memory, components, devices and media that retain digital computer data used for computing for some interval of time....
,
disk mirroring or
RAID1 is the replication of logical disk volumes onto separate physical
hard diskA hard disk drive is a non-volatile storage device that stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk...
s in
real timeIn computer science, real-time computing , or "reactive computing", is the study of hardware and software systems that are subject to a "real-time constraint"—i.e., operational deadlines from event to system response. By contrast, a non-real-time system is one for which there is no deadline, even...
to ensure continuous availability. A
mirrored volume is a complete logical representation of separate volume copies.
In a
Disaster RecoveryDisaster recovery is the process, policies and procedures related to preparing for recovery or continuation of technology infrastructure critical to an organization after a natural or human-induced disaster....
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
microcodeMicrocode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in a special high-speed memory and translates machine instructions into sequences of detailed...
on the
disk array controllerA disk array controller is a device which manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.A disk array controller name is...
or via
serverA server is an application running on a computer that delivers a service. For example, a web server will deliver web pages when requested by a browser . The way a server and a client dialogs is called a protocol...
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 ObjectiveRecovery Point Objective describes the acceptable amount of data loss measured in time.The Recovery Point Objective is the point in time to which you must recover data as defined by your organization. This is generally a definition of what an organization determines is an "acceptable loss" in a...
(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.
Mirroring is different from
file shadowingThere are several different technologies that use the term file shadowing, but the intent of file shadowing within these technologies is to provide an exact copy ....
..
Explanation
It is recognized that disks are an inherently unreliable component of computer systems. Mirroring is a technique to allow a system to automatically maintain multiple copies of data so that in the event of a disk hardware failure a system can continue to process or quickly recover data. Mirroring may be done locally where it is specifically to cater for disk unreliability, or it may be done remotely where it forms part of a more sophisticated disaster recovery scheme, or it may be done both locally and remotely, especially for high availability systems. Normally data is mirrored onto physically identical drives, though the process can be applied to logical drives where the underlying physical format is hidden from the mirroring process.
Typically mirroring is provided in either hardware solutions such as
disk arrayA disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...
s or in software within the operating system. As data is written to disk, the system automatically writes a second copy to one or more further locations. The system can either acknowledge that the data is safely written after one drive has confirmed a successful write operation or only when both drives have confirmed, depending on how critical data integrity is, there is normally a performance advantage in not waiting for the second write operation. Data can be read from either disk, and a high performance system would recognize which disk was in a better physical state to retrieve the data most quickly.
There are several scenarios for what happens when a disk fails. In a hot swap system, in the event of a disk failure, the system itself typically diagnoses a disk failure and signals a failure. Sophisticated systems may automatically activate a hot standby disk and use the remaining active disk to copy live data onto this disk. Alternatively, a new disk is installed and the data is copied to it. In less sophisticated systems, the system is operated on the remaining disk until such time as a spare disk can be installed with minimum disruption.
The copying of data from one pair of a mirror to another is sometimes called
resilvering though more commonly it is simply known as rebuilding. During the rebuilding process, system performance is usually degraded as the disk system is fully occupied in copying data from one disk to the other.
It is often misunderstood that mirroring of disks is a substitute for taking regular backups as it is incorrectly assumed that the only cause of data loss is disk failure. In fact the most trivial of user actions can delete data which then would need to be recovered, and in commercial operations it is far more likely that backups are used to recover from processing errors, user mistakes or vandalism, all of which are not protected by mirroring.
Mirroring can be performed site to site either by rapid data links, for example fibre optic links, which over distances of 500m or so can maintain adequate performance to support real-time mirroring. Longer distances or slower links maintain mirrors using an asynchronous copying system. For remote disaster recovery systems, this mirroring may not be done by integrated systems but simply by additional applications on master and slave machines.
It is differentiated from a snapshot in that there are no remaining links between the original (or
source) and the copy (or
mirror).
Other benefits of mirroring
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.
Other schemes
Some mirroring schemes employ three disks, with two of the disks for the redundancy mirroring and the third to be split off for performing backups. In
EMCEMC Corporation is a U.S. Fortune 500 and S&P 500 provider of information infrastructure systems, software and services. It is headquartered in Hopkinton, Massachusetts, USA. Its flagship product, the Symmetrix, is the foundation of storage networks in many large data centers.-History:EMC was...
nomenclature, the third disk is called a Business Continuance Volume (BCV).
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