Hammer: Transactional file updates

[Joerg Sonnenberger]

On Tue, Aug 19, 2008 at 07:00:23PM -0700, Matthew Dillon wrote:
>     * Unix in general does not guarantee atomicy between mmap-read or
>       mmap-written blocks and read() or write() ops.  This is because
>       it has no way to know what the user program actualy wants when,
>       since all memory accesses are page-based.

Unix in practise *does* guarantee this. The only exception of an
Operating System still in use is OpenBSD. It is non-trivial to do, e.g.
in NetBSD it was done with the switch to the Unified Buffer Cache,
FreeBSD did as you very likely know did it by changes to the Buffer
Cache etc. Otherwise I agree.

Joerg