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Date:   Wed, 10 Mar 2021 13:55:31 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Neal Gompa <ngompa13@gmail.com>
Cc:     Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-nvdimm@lists.01.org,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        darrick.wong@oracle.com, dan.j.williams@intel.com, jack@suse.cz,
        viro@zeniv.linux.org.uk, Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, david@fromorbit.com, hch@lst.de,
        rgoldwyn@suse.de
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax
Message-ID: <20210310135531.GP3479805@casper.infradead.org>
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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@infradead.org> 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.