Disk mirroring
Encyclopedia
In data storage
, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical hard disk
s 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.
Mirroring is different from file shadowing
.
Typically mirroring is provided in either hardware solutions such as disk array
s or in software within the operating system.
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, none of which are protected against 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).
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.
nomenclature, the third disk is called a Business Continuance Volume (BCV).
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
s in real time
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
to ensure continuous availability
Continuous Availability
Continuous Availability is an approach to computer system and application design that protects users against downtime, whatever the cause and ensures that users remain connected to their documents, data files and business applications. Continuous availability describes the information technology...
. A mirrored volume is a complete logical representation of separate volume copies.
In a Disaster Recovery
Disaster recovery
Disaster 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. Disaster recovery is a subset of business continuity...
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
Microcode
Microcode 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 special high-speed memory and translates machine instructions into sequences of detailed...
on the disk array controller
Disk array controller
A 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 server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
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
Recovery point objective
-Recovery point objective :When computers used for normal "production" business services are affected by a "Major Incident" that cannot be fixed quickly, then the Information Technology Service Continuity Plan is performed, by the ITSC recovery team...
(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 shadowing
File shadowing
There 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 array
Disk array
A 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.
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, none of which are protected against 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
EMC Corporation , a Financial Times Global 500, Fortune 500 and S&P 500 company, develops, delivers and supports information infrastructure and virtual infrastructure hardware, software, and services. EMC is headquartered in Hopkinton, Massachusetts, USA.Former Intel executive Richard Egan and his...
nomenclature, the third disk is called a Business Continuance Volume (BCV).