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Date:   Sat, 13 Mar 2021 23:00:56 +0100
From:   Adam Borowski <kilobyte@angband.pl>
To:     Neal Gompa <ngompa13@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Goldwyn Rodrigues <rgoldwyn@suse.de>,
        Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Jan Kara <jack@suse.cz>, Al Viro <viro@zeniv.linux.org.uk>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, david <david@fromorbit.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax
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On Sat, Mar 13, 2021 at 11:24:00AM -0500, Neal Gompa wrote:
> On Sat, Mar 13, 2021 at 8:09 AM Adam Borowski <kilobyte@angband.pl> wrote:
> >
> > On Wed, Mar 10, 2021 at 02:26:43PM +0000, Matthew Wilcox wrote:
> > > On Wed, Mar 10, 2021 at 08:21:59AM -0600, Goldwyn Rodrigues wrote:
> > > > DAX on btrfs has been attempted[1]. Of course, we could not
> > >
> > > But why?  A completeness fetish?  I don't understand why you decided
> > > to do this work.
> >
> > * xfs can shapshot only single files, btrfs entire subvolumes
> > * btrfs-send|receive
> > * enumeration of changed parts of a file
> 
> XFS cannot do snapshots since it lacks metadata COW. XFS reflinking is
> primarily for space efficiency.

A reflink is a single-file snapshot.

My work team really wants this very patchset -- reflinks on DAX allow
backups and/or checkpointing, without stopping the world (there's a single
file, "pool", here).

Besides, you can still get poor-man's whole-subvolume(/directory)
snapshots by manually walking the tree and reflinking everything.
That's not atomic -- but rsync isn't atomic either.  That's enough for
eg. dnf/dpkg purposes.


Meow!
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