Disk duplexing is a variation of disk mirroring in which each of multiple storage disks has its own SCSI controller. Disk mirroring (also known as RAID-1) is the practice of duplicating data in separate volumes on two hard disks to make storage more fault-tolerant. Mirroring provides data protection in the case of disk failure, because data is constantly updated to both disks. However, since the separate disks rely upon a common controller, access to both copies of data is threatened if the controller fails. Disk duplexing overcomes this problem; the use of redundant controllers enables continued data access as long as one of the controllers continues to function.
This failover method helps to ensure that data access will continue transparently to the user and allows technicians to take the server down to replace the defective controller at a more opportune time, instead of at the moment of failure. The ability to choose when the server comes down can be very advantageous, because -- in accordance with Murphy's Laws of Information Technology (Law of Inconvenient Malfunction) -- a device is likely to fail at the least opportune possible moment. Nevertheless, some experts advocate other systems (such as higher level RAID configurations) that don't require taking the server down to replace defective hardware.
Another benefit of disk duplexing is increased throughput. Using a technique known as a split seek, whichever disk can deliver the requested data more quickly responds. Multiple requests may also be split between the disks for simultaneous processing.
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