[Dirvish] Dirvish/rsync thinking partition is 'new'

[Adrian von Bidder]

On Sunday 06 March 2011 13.28:29 Paul Slootman wrote:
> Seems a bit on the assinine side to duplicate say, an
> 
> > unchanged 100GB iso image, just because the user 'touch'd, 
> > chmod'ed or chown'ed it. Really a rather serious waste of
> > resources.

There's no choice here: hardlinked files have the same inode, which means if 
the inode changes, the file has to be duplicated. There shouldn't be a need 
to retransmit it - if that's currently the case it should be fixed.

> I don't know whether the COW implementation on top of btrfs might help
> in that respect.

It depends on how rsync works, exactly, but from the btrfs side there's no 
need for the file data to be duplicated: the directory with the "inode" 
information [ I understand there's no inode in the traditional sense, but 
the ownership and mode information is obviously stored somewhere ] is 
duplicated, but the two copies of the file point to the same data on disk.  
Haven't tested this scenario though, so maybe rsync does something "stupid" 
that causes the file to be duplicated.

The dirvish cow patch just removes the "--link-dest" option and adds "--
inplace" to the rsync call.

cheers
-- vbi



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