On Tue, 29 Jul 2003, Alastair Reid wrote:
Ben Rudiak-Gould wrote:
If it's not possible to provide a guarantee of File identity then we should probably drop the whole idea of File values.
I haven't been following in enough detail to know if File values are a good idea or not but it's maybe worth mentioning that if you're NFS, there's no way to test file identity. IIRC, the problem is that a machine might nfs-export overlapping parts of its filesystem so you might have two names for the same file.
I don't think this is fatal. The important part of File identity is that two File values which compare equal necessarily denote the same file, not the converse. I don't think there's any observable difference between File values which denote different files and File values which denote the same file but don't compare equal, since there could be a demon (not daemon -- I'm thinking Maxwell's demon here) watching everything you do to file x/y and immediately doing the same thing to file y/x. Now, if NFS doesn't even guarantee the forward implication (for e.g. filehandle comparison by value), then that's bad. But not bad for us. Bad for anyone foolish enough to use NFS.
ps (At least in FreeBSD) mmapping an NFS-mounted file can also lead to unhappiness. If you mmap the same NFS-mounted file multiple times, you can end up with multiple copies of the same page which leads to chaos when you start writing to the pages.
That's bad too. See previous paragraph. :-) -- Ben