Re: About in-band dedupe for v4.7

[David Sterba <dsterba@suse.cz>]

On Tue, May 10, 2016 at 03:11:19PM -0700, Mark Fasheh wrote:
> On Tue, May 10, 2016 at 03:19:52PM +0800, Qu Wenruo wrote:
> > Hi, Chris, Josef and David,
> > 
> > As merge window for v4.7 is coming, it would be good to hear your
> > ideas about the inband dedupe.
> > 
> > We are addressing the ENOSPC problem which Josef pointed out, and we
> > believe the final fix patch would come out at the beginning of the
> > merge window.(Next week)
> 
> How about the fiemap performance problem you referenced before? My guess is
> that it happens because you don't coalesce writes into anything larger than
> a page so you're stuck deduping at some silly size like 4k. This in turn
> fragments the files so much that fiemap has a hard time walking backrefs.
> 
> I have to check the patches to be sure but perhaps you can tell me whether
> my hunch is correct or not.
> 
> 
> In fact, I actually asked privately for time to review your dedupe patches,

You can also ask for time to review publically, with me in CC.

> but I've been literally so busy cleaning up after the mess you left in your
> last qgroups rewrite I haven't had time.
> 
> You literally broke qgroups in almost every spot that matters. In some cases
> (drop_snapshot) you tore out working code and left in a /* TODO */ comment
> for someone else to complete.  Snapshot create was so trivially and
> completely broken by your changes that weeks later, I'm still hunting a
> solution which doesn't involve adding an extra _commit_ to our commit.  This
> is a MASSIVE regression from where we were before.

That's unfortunatelly true and I take part of the blame for allowing
that.