File backup in linux with advisory locks

How do backup programs make开发者_C百科 sure they get a consistent copy of a file, when file locks in linux mostly are advisory?

For example if some other process do not respect file locks and writes to a file, how can I create a consistent copy of that file?

This is quite an interesting topic, the modern way seems to be to use a filesystem snapshot; another way is to use a block-device snapshot.

In any case, some kind of snapshot is the best solution. Zfs has snapshots (but is not available as a "first class" filesystem under Linux), as does btrfs (which is quite new).

Alternatively, a LVM volume can have a block-level snapshot taken (which can then be mounted readonly in a temporary location while a backup is taken).

If you had mandatory file locks, then a backup program would disrupt normal operation of (for example) a database so that it was not able to work correctly. Moreover, unless there was a mechanism to atomically take a mandatory lock on every file in the filesystem, there would be no way to take a consistent backup (i.e. with every file as it was at the same moment).