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

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Date:   Wed, 10 Mar 2021 13:02:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Neal Gompa <ngompa13@...il.com>
Cc:     Shiyang Ruan <ruansy.fnst@...itsu.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-nvdimm@...ts.01.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        darrick.wong@...cle.com, dan.j.williams@...el.com, jack@...e.cz,
        viro@...iv.linux.org.uk, Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
        ocfs2-devel@....oracle.com, david@...morbit.com, hch@....de,
        rgoldwyn@...e.de
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax

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.