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Data Mirroring

Data mirroring refers to the real-time operation of copying data, as an exact copy, from one location to a local or remote storage medium. In computing, a mirror is an exact copy of a dataset. Most commonly, data mirroring is used when multiple exact copies of data are required in multiple locations.

data mirroring

Data mirroring can also be accomplished through disk mirroring, which involves making exact copies of data on different partitions of the same disk or on separate disks, all within the same system. With separate systems (meaning each system has at least a separate hard drive controller card), the process is called disk duplexing. Data mirroring is particularly useful for a rapid disaster recovery.

Data mirroring is the process of replicating the contents of files that have been transferred into or modified within USB devices. Once mirror copies are made, they can be maintained in a password-protected network share. This share folder is best kept in a remote location to save space on local machines. This way, it can be securely accessed and analyzed by only the administrator or trusted users.

Data mirroring software is a tool that you can use to configure policies to ensure that data mirroring happens whenever any file action is carried out on a USB, or just when specific file actions are performed. Details regarding the file operation such as file name, users, devices, endpoints involved as well as the time of action can also be recorded.

Although data mirroring requires considerable bandwidth and disk space, when configured carefully, it is an effective way of safeguarding corporate data that has been transferred out of the network using removable devices.

Data mirroring is often confused with data replication- a process intrinsic to database management systems. While there can be multiple uses for data replication in an enterprise, such as making data fluid and readily accessible in larger networks, the term database mirroring is specially attributed towards disaster recovery and prevention against hard-deletion of files.

If and when circumstances do arise where information is lost due to data theft or some other human or hardware-related error after being transferred, the data mirroring software ensures that the share folder can still be utilized. It can be cross-referenced to identify the exact contents of the files that have been compromised. If the information includes crucial details such as passwords, financial records, or even protected personal information (PPI), remediation efforts can be carried out immediately.

It is crucial that data required for critical tasks or data accessed by many employees remains in tact. It is equally important to have the information always available to authorized employees so that they can access it whenever necessary.

However, if data loss occurs due to neglect or theft during file actions involving USB devices, having a data mirroring software in place provides the organization with a cost-effective way to recover. After extracting information from a system, whatever files become corrupted or go missing while being transported in the USB device can be swiftly retrieved from the network share and restored to a location where relevant employees can regain access to them.

In data storage, disk mirroring is the replication of logical disk volumes onto separate physical hard disks in real time to ensure continuous availability. It is most commonly used in RAID 1. 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 data storage device 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.

Typically, mirroring is provided in either hardware solutions such as disk arrays, or in software within the operating system (such as Linux mdadm and device mapper).[1][2] Additionally, file systems like Btrfs or ZFS provide integrated data mirroring.[3][4] There are additional benefits from Btrfs and ZFS, which maintain both data and metadata integrity checksums, making themselves capable of detecting bad copies of blocks, and using mirrored data to pull up data from correct blocks.[5]

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 a spare disk can be installed.

Mirroring can be performed site to site either by rapid data links, for example fibre optic links, which over distances of 500 m 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 primary and secondary machines.

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.

Database mirroring is a solution for increasing the availability of a SQL Server database. Mirroring is implemented on a per-database basis and works only with databases that use the full recovery model.

In the event of a disaster, in high-safety mode with automatic failover, failover quickly brings the standby copy of the database online (without data loss). In the other operating modes, the database administrator has the alternative of forcing service (with possible data loss) to the standby copy of the database. For more information, see Role Switching, later in this topic.

Database mirroring provides complete or almost complete redundancy of the data, depending on whether the operating mode is high-safety or high-performance. For more information, see Operating Modes, later in this topic.

A database mirroring partner running on SQL Server 2008 Enterprise or later versions automatically tries to resolve certain types of errors that prevent reading a data page. The partner that is unable to read a page requests a fresh copy from the other partner. If this request succeeds, the unreadable page is replaced by the copy, which usually resolves the error. For more information, see Automatic Page Repair (Availability Groups: Database Mirroring).

To minimize downtime for a mirrored database, you can sequentially upgrade the instances of SQL Server that are hosting the failover partners. This will incur the downtime of only a single failover. This form of upgrade is known as a rolling upgrade. For more information, see Upgrading Mirrored Instances.

automatic failoverThe process by which, when the principal server becomes unavailable, the mirror server to take over the role of principal server and brings its copy of the database online as the principal database.

High-performance modeThe database mirroring session operates asynchronously and uses only the principal server and mirror server. The only form of role switching is forced service (with possible data loss).

manual failoverA failover initiated by the database owner, while the principal server is still running, that transfers service from the principal database to the mirror database while they are in a synchronized state.

After a mirroring session starts or resumes, the process by which log records of the principal database that have accumulated on the principal server are sent to the mirror server, which writes these log records to disk as quickly as possible to catch up with the principal server.

WitnessFor use only with high-safety mode, an optional instance of SQL Server that enables the mirror server to recognize when to initiate an automatic failover. Unlike the two failover partners, the witness does not serve the database. Supporting automatic failover is the only role of the witness.

Database mirroring maintains two copies of a single database that must reside on different server instances of SQL Server Database Engine. Typically, these server instances reside on computers in different locations. Starting database mirroring on a database, initiates a relationship, known as a database mirroring session, between these server instances.

One server instance serves the database to clients (the principal server). The other instance acts as a hot or warm standby server (the mirror server), depending on the configuration and state of the mirroring session. When a database mirroring session is synchronized, database mirroring provides a hot standby server that supports rapid failover without a loss of data from committed transactions. When the session is not synchronized, the mirror server is typically available as a warm standby server (with possible data loss). 041b061a72

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