sftp locks can get stuck

[Jan Hudec]

On Wed, Jan 04, 2006 at 19:33:45 +1100, Robert Collins wrote:
> On Wed, 2006-01-04 at 09:19 +0100, Jan Hudec wrote:
> 
> > The directory renaming scheme would be: create a uniquely named
> > directory, create a uniquely named file in it, rename the directory to
> > the lock and look whether it contains your uniquely named file (again,
> > return value of network operation is never reliable).
> 
> Actually, arch renamed an *existing* dir to take out the lock, the
> theory being that NO fs on the planet is silly enough to let two people
> rename 'foo' to 'foo-something-unique' and have both succeed. (whereas
> its conceivable that two renames could both succeed, one to the dir and
> one into the dir.

Yes. But both schemes do work. The test whether a particular, uniquely-named
file exists within the target directory would fail if it was renamed inside
that directory instead.

> > I can perhaps think of a commit schema (one where transaction can be
> > forced to roll back in a way the writer won't damage anything), but it
> > might cause readers to fail.
> > 
> > I know you turned down the idea of grouping data (in knits - won't work
> > for weaves) by revision instead of by file for performance reasons. But
> > it would have one advantage -- transactions would be easy.
> 
> Actually, because of the need to annotate historical data, it is not as
> easy as I think you think it would be - arch had massive problems due to
> that - it was only transactional for part of its operation.
> Transactional and isolated files are really very orthogonal - look at
> most fast, commercial grade databases for instance.

Yes. The question is, whether their requirements on storage can be satisfied
by a network filesystem as bad as ftp or nfs.

-- 
						 Jan 'Bulb' Hudec <bulb at ucw.cz>
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