One billion files on Linux

This use of rsync presents an interesting case for the userspace portion of checkpoint/restart.

During checkpoint we often need to checkpoint the contents of the filesystems. One way to do that is with a frozen filesystem and rsync. Obviously if we're rsync'ing to mirror the filesystem in the first place then we shouldn't attempt to checkpoint the rsync task's filesystem(s) with rsync -- we'd want to do a "local" snapshot if possible.

Since the kernel does not force userspace to save the filesystem contents userspace can choose if and how it will do so. In other words this case requires no special changes to the checkpoint syscall.