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

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Date:   Wed, 10 Mar 2021 16:53:15 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Neal Gompa <ngompa13@...il.com>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
        ocfs2-devel@....oracle.com, david <david@...morbit.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax

On Wed, Mar 10, 2021 at 6:27 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Mar 10, 2021 at 08:21:59AM -0600, Goldwyn Rodrigues wrote:
> > On 13:02 10/03, Matthew Wilcox 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.
> >
> > 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.

Isn't DAX useful for pagecache minimization on read even if it is
awkward for a copy-on-write fs?

Seems it would be a useful case to have COW'd VM images on BTRFS that
don't need superfluous page cache allocations.