filesystem boundary

The line where one mounted filesystem ends and another begins. Everything hangs off a single /, so the tree looks like one disk. It is not: /home on its own partition, an attached volume, /proc, /sys, a network share — each is a separate filesystem grafted onto the same tree. Crossing one changes what a command does. Inside a filesystem mv rewrites a directory entry; across a boundary it copies the data and then deletes the original, which is slow, can fail halfway on a full disk, and gives the file a new inode number. A search starting from / walks into /proc, /sys and network mounts unless you add -xdev, which keeps find inside one filesystem. df ~/site ~/backup tells you which side of a boundary each path is on, before you type the command.