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The simplest form of redundancy is mirroring. In mirroring, there exist two copies of each page. When the client swaps out a page, the page is sent to two different servers. Even when one of the servers crashes, the application is able to complete its execution, because all pages of the crashed server exist on the mirror servers. Obviously the crash recovery overhead, in case of mirroring, is minimal. However, the runtime overhead is rather high, since each pageout requires two page transfers. To make matters worse, mirroring wastes half of the remote memory used.

Evangelos Markatos
Wed Aug 7 11:36:29 EET DST 1996