Risks of backing up live mounted filesystems using dump(8)

[Alan Cox]

Journals make the problem far worse.

On restore you will restore a journal log no longer related to whats on
the media, then risk replaying it and causing further damage. 

Block reallocation is also nasty with a dump done that way because you
may end up with undetected data corruption including leaks of data
between uids.

A file system based backup is a good deal safer.

You should also know if it works or not. Sct (one of the ext2 designers)
always said there were three approaches to backups

- "I ought to do it"   (people who've not had a catastrophic failure)
- "We back it up" (people who've not had a catastrophic restore failure)
- "We back it up and test the backup/restore process
  regularly" (battle scarred veterans)