Re: [PATCH v7 00/47] xfs: add reverse mapping support

Darrick J. Wong
2016-08-05T00:49:33Z

On Fri, Aug 05, 2016 at 09:50:15AM +1000, Dave Chinner wrote:
> On Thu, Aug 04, 2016 at 08:48:45AM -0700, Darrick J. Wong wrote:
> > On Wed, Aug 03, 2016 at 07:18:52PM -0700, Mark Fasheh wrote:
> > > On Wed, Aug 03, 2016 at 05:58:43PM -0700, Darrick J. Wong wrote:
> > > > On Wed, Aug 03, 2016 at 01:55:20PM -0700, Darrick J. Wong wrote:
> > > > > On Wed, Aug 03, 2016 at 12:45:36PM -0700, Mark Fasheh wrote:
> > > > > > Where can I the patches to enable dedupe_range on xfs? I tested your
> > > > > > previous devel branch based on Linux v4.7-rc3 with duperemove
> > > > > > (https://github.com/markfasheh/duperemove) and it worked extremely well -
> > > > > > even handling some cases that btrfs still has issues with. I actually
> > > > > > committed the code to enable xfs support in duperemove so anyone can test on
> > > > > > xfs with the dedupe_range patches.
> > > > > >
> > > > > > I'd gladly test your latest patches by doing my usual 'large' duperemove
> > > > > > tests once I can get ahold of the dedupe_range work :)
> > > > >
> > > > > Your best bets are probably the -experimental trees:
> > > > > https://github.com/djwong/linux/commits/djwong-experimental
> > > > > https://github.com/djwong/xfsprogs/commits/djwong-experimental
> > > > >
> > > > > I haven't updated them in a while because I've been busy trying
> > > > > to get reverse-mapping (the start of those patchbombs) into 4.8.
> > > > >
> > > > > Just as a warning, don't put anything critical on those XFS filesystems
> > > > > because there's going to be a disk format update between now and the
> > > > > next time I post the patches because Dave and I decided to cache the
> > > > > block counts for the new btrees in order to speed up mounting. I don't
> > > > > anticipate having time to clean up my dev tree and push to github until
> > > > > a week or two after the merge window closes.
> > > >
> > > > That said, all the craziness from the last two weeks (xfs_scrub sprint
> > > > and the rmapbt review fixes) are now in the -wtf tree, which /should/
> > > > behave. I've dumped everything there in completely not cleaned up
> > > > format, but this does have the AGF btree block counter stuff I talked
> > > > about above.
> > > >
> > > > https://github.com/djwong/linux/commits/djwong-wtf
> > > > https://github.com/djwong/xfsprogs/commits/djwong-wtf
> > >
> > > Fantastic. Don't worry about corrupting data, I've been doing this long
> > > enough to know to use this only on scratch partitions for now :) I'm in the
> > > middle of a couple other tests but once I'm done there I'll grab those
> > > branches and give them another spin. Typically I'm running through 800G-1TB
> > > on these (with varying duplicated data percentages).
> > >
> > > Silly question which I could just answer by looking at the patches - are you
> > > reporting FIEMAP_EXTENT_SHARED yet for extents with more than one owner? We
> > > use that flag (by comparing SHARED bytes before and after dedupe) to give
> > > people an estimate of how much space was saved. I presume figuring the
> > > shared state of an extent would be as easy as querying the rmap btree,
> > > correct?
> >
> > Yes, it should report FIEMAP_EXTENT_SHARED. In the past it would
> > report exactly which parts of an extent were shared. If a file mapped
> > blocks 30-40 and block 35 was shared, it would report 30-34, 35
> > (shared), and 36-40. However, btrfs only reports a single extent
> > "30-40 (shared)" so I stopped doing that.
>
> I'd much prefer that fiemap gives exact information about shared
> extents. FIEMAP is a diagnostic tool and as such we need it to
> accurately reflect the exact extent map of the inode being queried
> so we aren't mislead about the layout of the file during trouble
> shooting.
>
> Hence I think what btrfs does is sub-optimal and we shouldn't copy
> it. Let's do the right thing with XFS and if people complain it is
> different to btrfs then someone can go make btrfs report an accurate
> extent map, too.

That's probably going to be right after I get a xfstest committed that
checks for the XFS behavior and fails on btrfs. :)

--D

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com