>Of course, while you copy the beginning of the file, the end may be altered, and vice versa. This is quite risky. On the other hand, it is quite possible that a user has a file open, but is not actively changing it - for this reason, at BAta/Bolivia we do backups at noon (against potentially open files), but these are a secondary backup, "just in case". The main backup is done around midnight.
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>I believe that to have a "real" hot-backup feature would require a database server that offers precisely this capability.

Yes... it would require that all buffers at all workstations be flushed, and would stop (or queue) all the transactions while it creates the backup. It would still continue the read operations, though.