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From:   Neal Gompa <ngompa13@gmail.com>
Date:   Thu, 11 Mar 2021 03:26:11 -0500
Message-ID: <CAEg-Je9e1R2NAqtZfryM99+Z98SGjxTSQjt-CMyKRMxvDwtsyg@mail.gmail.com>
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax
To:     Dan Williams <dan.j.williams@intel.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>
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On Wed, Mar 10, 2021 at 7:53 PM Dan Williams <dan.j.williams@intel.com> wro=
te:
>
> On Wed, Mar 10, 2021 at 6:27 AM Matthew Wilcox <willy@infradead.org> wrot=
e:
> >
> > 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 featur=
e
> > > >
> > > > 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 thi=
ngs.
> > > > DAX is about giving raw access to the hardware.  btrfs is about off=
ering
> > > > extra value (RAID, checksums, ...), none of which can be done if th=
e
> > > > filesystem isn't in the read/write path.
> > > >
> > > > That's why there's no DAX support in btrfs.  If you want DAX, you h=
ave
> > > > 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.

I could also see this being useful for databases (and maybe even swap
files!) on Btrfs, if I'm understanding this feature correctly.


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=E7=9C=9F=E5=AE=9F=E3=81=AF=E3=81=84=E3=81=A4=E3=82=82=E4=B8=80=E3=81=A4=EF=
=BC=81/ Always, there's only one truth!