Re: [PATCH] cowlinks v2

[Pavel Machek]

Hi!

> > > get_data_id() is one way to detect equivalent files.  Another would be
> > > a function files_equal(fd1, fd2) which returns a boolean.
> > 
> > files_equal(...) would lead to quadratic number of calls, no?
> > 
> > > get_data_id() has the advantage that it can report immediately whether
> > > a file has _any_ cowlink peers, which is important for programs that
> > > scan trees.  Perhaps getxattr() would be reasonable interface, using a
> > > named attribute "data-id".
> > 
> > Yes, get_data_id() is extremely ugly name.
> 
> I think it is worth asking if we really want to give userspace a way of
> doing this or not.  It exposes fairly low level FS details to userspace,
> and this will limit our ability to change the implementation of the FS
> in the future (partially shared files?).  Certainly there has been some
> pain caused over the years because userspace can ask for the inode number,
> and people have written file systems which do not use inodes.  Then they
> have to kluge around this and make something up.  I would hate to see
> us implement an interface that causes long term pain.

It should not be painfull, get_data_id() can allways return
-Esomething, meaning "I do not know".

> I also cant really think of anyone who would need this information.  I have
> seen diff and tar used as examples.  Perhaps diff would run faster but that
> seems like a very special case thing, and diff will certainly work w/o it.

Speeding up diff was one of main cowlinks motivations.
									Pavel
-- 
Horseback riding is like software...
...vgf orggre jura vgf serr.
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