A tyte of backup where you capture the entire set of files as they were at one
instant in time. The problem with taking these sorts of backup is they mean shutting
down all program for hours while you copy all the files. The backups are voluminous
since you backup not only your own files, but system and program files. Sun Solaris
ZFS file system has a snapshot technique that is almost instantaneous. I believe it
is more than just a disk to disk copy. I suspect that files are effectively made read
only and copied only as needed later if the original file changes.