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Disk Mirroring
Disk mirroring, sometimes referred to as RAID 1, produces a
bit-for-bit copy of data from a primary disk drive to a secondary disk drive. If either disk
drive fails, the other disk drive provides continued access to all data. RAID 1 is typically
performed within a single localized disk subsystem.
Disk mirroring is often employed with the primary and secondary
disk drive separated by a campus or metropolitan distance. Data can be replicated
synchronously or asynchronously.
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Synchronous
mirroring keeps both the local and remote disk up to date on every write request, which can cause reduced overall application
performance. To minimize this impact of disk mirroring on application performance, high
bandwidth, low-latency network connections like Fibre Channel, DWDM, or CWDM are
employed and are limited to campus/metro distances.
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Asynchronous
mirroring allows multiple writes to be queued for replication to the remote site. In the event of a failure of the local disk drive, some number of write requests may not have been completed to the remote drive, so the
application may lose some of the recent history when it fails over to the remote drive.
Disk mirroring is an ideal solution for applications that demand transactional high availability and transparent failover either when the local disk subsystem fails or to expedite recovery after a site disaster.
Disk mirroring does not protect against accidentally deleted or corrupted files because whatever is written to the primary disk is immediately copied to its mirrored disk. Deleted files on the primary disk are instantly deleted on the mirrored disk, and
files corrupted on the primary disk are instantly corrupted on the mirrored disk. Therefore,
mirroring does not eliminate the need for periodic backups of data to another storage medium like magnetic
tape. Periodic backups of data create historical versions so that, when a file is deleted or
corrupted on the mirrored drive pair, a copy can be restored from a point in time before the deletion or
corruption occurred.
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