Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax

[Matthew Wilcox]

On Wed, Mar 10, 2021 at 08:36:06AM -0500, Neal Gompa wrote:
> On Wed, Mar 10, 2021 at 8:02 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote:
> > > Forgive my ignorance, but is there a reason why this isn't wired up to
> > > Btrfs at the same time? It seems weird to me that adding a feature
> >
> > btrfs doesn't support DAX.  only ext2, ext4, XFS and FUSE have DAX support.
> >
> > If you think about it, btrfs and DAX are diametrically opposite things.
> > DAX is about giving raw access to the hardware.  btrfs is about offering
> > extra value (RAID, checksums, ...), none of which can be done if the
> > filesystem isn't in the read/write path.
> >
> > That's why there's no DAX support in btrfs.  If you want DAX, you have
> > to give up all the features you like in btrfs.  So you may as well use
> > a different filesystem.
> 
> So does that mean that DAX is incompatible with those filesystems when
> layered on DM (e.g. through LVM)?

Yes.  It might be possible to work through RAID-0 or read-only through
RAID-1, but I'm not sure anybody's bothered to do that work.

> Also, based on what you're saying, that means that DAX'd resources
> would not be able to use reflinks on XFS, right? That'd put it in
> similar territory as swap files on Btrfs, I would think.

You can use DAX with reflinks because the CPU can do read-only mmaps.
On a write fault, we break the reflink, copy the data and put in a
writable PTE.