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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. On a broadcast network like Ethernet or FDDI, each page is automatically broadcasted to all remote memory servers, thus eliminating the need to explicitly send each page both to the main and the mirror server. Nevertheless, mirroring wastes half of the remote memory used.

Evangelos Markatos
Fri Mar 24 14:41:51 EET 1995