A computer system implements hot mirroring for main system memory. That is, the computer system permits a user to hot plug a new memory board into the system and the system will respond by switching to a mirrored memory mode in which write cycles are performed to both memory boards (new and old). Once a new board is hot plugged into the system, the contents of the old board are copied over, in a background mode, to the new board so that both boards will have the same data. Because this background copying process may take a non-trivial amount of time and may detrimentally interfere with other system traffic, the system a user to exert control over the relative speed of the background copying so as to trade-off the time it takes to switch over to the mirroring mode versus the impact on on-going system behavior.