Re: Data Deduplication with the help of an online filesystem check

[Chris Mason <chris.mason@oracle.com>]

On Tue, 2009-04-28 at 17:49 +0400, Andrey Kuzmin wrote:
> On Tue, Apr 28, 2009 at 2:02 PM, Chris Mason <chris.mason@oracle.com> wrote:
> > On Tue, 2009-04-28 at 07:22 +0200, Thomas Glanzmann wrote:
> >> Hello Chris,
> >>
> >> > There is a btrfs ioctl to clone individual files, and this could be used
> >> > to implement an online dedup.  But, since it is happening from userland,
> >> > you can't lock out all of the other users of a given file.
> >>
> >> > So, the dedup application would be responsible for making sure a given
> >> > file was not being changed while the dedup scan was running.
> >>
> >> I see, does that mean that I can not do ,,dedup'' for files that are
> >> currently opened by a userland program?
> >
> > No, but it does mean the dedup done from userland is racey.  Picture
> 
> Race disappears if (background) dedupe is run against snapshot(s).
> 

True, but then you're only changing the blocks pointed to by the
snapshot.  The 'master' copy still points to the unduplicated blocks.

-chris