(Sorry if this is completely the wrong place to report this. I found no other obvious bug tracker to report this to and several other equally unlikely places to report it.) The URL to the "Snapshot Performance" in the device-mapper source (including current CVS) is dead, as it points to people.sistina.com. From here: http://sources.redhat.com/cgi-bin/cvsweb.cgi/device-mapper/INTRO?rev=1.5&content-type=text/x-cvsweb-markup&cvsroot=dm The 'snapshot' target supports asynchronous snapshots. See http://people.sistina.com/~thornber/snap_performance.html. I was able to retrieve it from the Wayback Machine: http://web.archive.org/web/20020718130631/http://people.sistina.com/~thornber/snap_performance.html
We merged most of the dm repo into the lvm2 repository and the INTRO file is no longer distributed in current updates. I've replaced it with the link you found, anyway.