This often improves performance because the majority of requests across a network are disk reads. In addition, a server with duplexed disk drives can read from one disk drive, write to a second, then, when the process is completed, create a mirror image of the most-recently stored data. After a disk or server has been taken out of operation and is ready to be put back online, a fault-tolerant system must provide a synchronization process that puts the redundant components back in sync with each other. All fault-tolerant products offer drawbacks, the most obvious of which are costs of the redundant hardware and the software to run the hardware. Another factor is overhead from the executable code that controls the mirroring or duplexing process.
Monetary costs are easy to figure out.
Determining whether such hardware and software costs are justifiable in any particular installation is a much more complex matter. This risk analysis process involves evaluating the loss potential, determining the equipment necessary to reduce the risk, and forming effective management procedures.