cvs commit: src/tools/regression/bin/mv regress.sh

[Diomidis Spinellis]

Alfred Perlstein wrote:
> Does 'mv' spawn off tar or something to do the copy part?
> 
> You can _usually_ get much better cross device performance
> by hooking two processes together like so:
> 
> tar -cf - -C /path/to/source . | tar -xf - -C /path/to/dst 
> 
> This will keep the disks a lot busier, but this can make
> things worse on cross device moves that happen to be
> on the same disk.
> 

This is an interesting idea.  Currently, mv spawns off cp and rm when 
rename fails with EXDEV.  Given that our cp often uses mmap to avoid the 
read/write context switching, I'm not sure that using a tar pipe will 
buy us a lot of performance.

Anyway, the major problem is now the semantics of POSIX for cross-device 
links.  The regression tests I wrote succeed in all rename(2)-based 
tests.  Running *the same tests* across devices on FreeBSD, Solaris, and 
Linux gives us three different sets of failed tests.  Also, a literal 
reading of POSIX seems to require mv to loose some data or leave data 
lying around when some parts of the cp/rm dance fail.  As bde@ pointed 
out in a private email, my initial implementation was intepreting POSIX 
too liberaly in this respect, trying to avoid data loss, so this also 
probably needs changing.

I'm discussing these aspects with bde@ off-list.  If anybody else wants 
to to be cc'd, drop me a line.

Diomidis