Re: [PATCH 0/2] btrfs: allow reflinks into NODATASUM files

[Qu Wenruo]

在 2026/7/13 00:36, Neal Gompa 写道:
> On Sun, Jul 12, 2026 at 10:25 AM Daan De Meyer via B4 Relay
> <devnull+daan.amutable.com@xxxxxxxxxx> wrote:
> > The primary use case for this series is building a NOCOW VM image from
> > individual partition images that are COW and checksummed. Today those
> > partitions cannot be cloned into the NODATACOW and NODATASUM destination,
> > so tools fall back to a full copy. This makes provisioning slower and
> > duplicates all of the image data up front.
> >
> > Allow cloning and deduplication from a checksummed file into a NODATASUM
> > file. The VM image can then share extents with the partition images.
> > Existing checksummed extents are COWed once when modified, protecting the
> > source checksums, while newly allocated extents use the destination's
> > normal NOCOW behavior.
> >
> > The reverse direction remains rejected because the destination would
> > expect checksums that do not exist.
> >
> > Patch 1 prevents swap activation from bypassing the COW protection when a
> > NODATASUM file references checksummed extents. Patch 2 relaxes the reflink
> > restriction in the safe direction.
> >
> > The xfstests branch is available at:
> > https://github.com/kdave/xfstests/pull/6
> >
> > Signed-off-by: Daan De Meyer <daan@xxxxxxxxxxxx>
> > ---
> > Daan De Meyer (2):
> >        btrfs: reject swapfile activation if any extent has checksums
> >        btrfs: allow reflinking from checksummed files into nodatasum files
> >
> >   fs/btrfs/inode.c   | 29 +++++++++++++++++++++++++++++
> >   fs/btrfs/reflink.c | 18 ++++++++++++++----
> >   2 files changed, 43 insertions(+), 4 deletions(-)
> > ---
> > base-commit: cab9e339cfbc1a4e075e53e281dfb00391e1a6bb
> > change-id: 20260712-reflink-into-nodatasum-aff962f3794e
>
> The patch series looks good to me.
>
> Reviewed-by: Neal Gompa <neal@xxxxxxxxx>

I appreciate your feedback as an end user/sys admin, but I doubt if you 
really understand the requirement or tried the patch by yourself.

I'd appreciate if you put more weight behind your reviewed-by tags.

> That said, I have a question: Is there no reason we couldn't reflink
> into a checksummed area and generate *new* checksums for that purpose?
> That would be useful for staging for regular backup.

Because btrfs' NODATASUM/NODATACOW flag is per-inode, as that's the only 
way we can store that flag.

If you really want proper per-extent NODATASUM/NODATACOW, there will 
need a huge change in the on-disk format, e.g. introducing new flags 
into btrfs_file_extent_item, and do the extra work handling corner cases 
like preallocation.

It's definitely not as simple two small patches.

> I can foresee that being useful for archiving OS trees or VM images as
> snapshots by creating checksums for the newly created snapshots...

Nope, one should not combine VM images with snapshots on btrfs.

Even if we have fixed all the direct IO problems, the extent bookend 
will waste a lot of space.

> --
> 真実はいつも一つ！/ Always, there's only one truth!