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

[Filipe Manana <fdmanana@kernel.org>]

On Thu, Jan 31, 2019 at 4:39 PM Filipe Manana <fdmanana@kernel.org> wrote:
>
> 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).

Another ping.
This is a problem that has to be solved one way or another.

Thanks.

>
> 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.