Re: [PATCH 3/4] Btrfs: check if destination root is read-only for deduplication

[Filipe Manana <fdmanana@kernel.org>]

On Thu, Dec 13, 2018 at 4:08 PM David Sterba <dsterba@suse.cz> wrote:
>
> On Wed, Dec 12, 2018 at 06:05:58PM +0000, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > Checking if the destination root is read-only was being performed only for
> > clone operations. Make deduplication check it as well, as it does not make
> > sense to not do it, even if it is an operation that does not change the
> > file contents (such as defrag for example, which checks first if the root
> > is read-only).
>
> And this is also change in user-visible behaviour of dedupe, so this
> needs to be verified if it's not breaking existing tools.

Have you had the chance to do such verification?

This actually conflicts with send. Send does not expect a root/tree to
change, and with dedupe on read-only roots happening
in parallel with send is going to cause all sorts of unexpected and
undesired problems...

This is a problem introduced by dedupe ioctl when it landed, since
send existed for a longer time (when nothing else was
allowed to change read-only roots, including defrag).

I understand it can break some applications, but adding other solution
such as preventing send and dedupe from running in parallel
(erroring out or block and wait for each other, etc) is going to be
really ugly. There's always the workaround for apps to set the
subvolume
to RW mode, do the dedupe, then switch it back to RO mode.

Thanks.