Re: [9fans] mv on directory

[Roman Shaposhnik <rvs@sun.com>]

On Nov 1, 2008, at 9:30 AM, Josh Wood wrote:
> All that said, it's not like I've never cursed a directory that
> wouldn't mv for me in Plan 9 -- so if someone had an answer for
> Rob's question: "What should mv do to a tree that resides on
> multiple file servers?", it could be interesting to discuss. I don't
> think arguing from rm -r is a good tact, though, because of the
> differing risk levels between a failed delete and a failed move. One
> might afford convenience in the former, and eschew it in the latter.

That's a very good point. UNIX in general does guarantee certain
things about rename of the
subdirectories within the same FS, but the price they pay in the
kernel and elsewhere
(NFS being the prime example) seems just too high (the original
explanation given by a
friend of mine who wrote this was much more colorful, but I guess guys
like IBM & co. cleaned up
kernel comments quite a bit ;-)):
    http://lxr.linux.no/linux+v2.6.27.4/fs/namei.c#L2479

The behavior of mv(1) as defined by POSIX seems to build on top of the
rename-within-the-same-FS
guarantee, which, in case of Plan9 is not applicable. Thus it would be
an interesting thought
exercise to go over the POSIX text:
     http://www.opengroup.org/onlinepubs/009695399/
and see how much of the required subdirectory move semantics could be
preserved even though
we lack one of the basic building blocks that makes it behave like it
does on UNIX.

Thanks,
Roman.