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

[David Sterba <dsterba@suse.cz>]

On Thu, Jan 31, 2019 at 04:44:39PM +0000, Hugo Mills wrote:
> On Thu, Jan 31, 2019 at 04:39:22PM +0000, Filipe Manana 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).
> > 
> > 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.
> 
>    Only if you want your incremental send chain to break on the way
> past...
> 
>    I think it's fairly clear by now (particularly from the last thread
> on this a couple of weeks ago) that making RO subvols RW and then back
> again is a fast way to broken incremental receives.

So, I think the way it should be fixed is to keep deduplication off the
read-only subvolumes. The main reason I see is to avoid the random
problems that arise from send + dedupe interaction. The cost is lower
deduplication ratio.

The main usecase being a primary subvolume with RO snapshots over time,
with optional incremental send. That I know is common and widely used.

A problematic usecase that would utilize deduplication over RO snapshots
does could be something like a set of subvolumes that have very similar
data, get snapshotted or set RO, followed by a dedupe pass.

To support the latter I'd go only via the RO->RW, dedupe, RW->RO way, as
this records that there was a change and does not collide with send
assumptions. That this leads to loss of incremental send must be
communicated to the user with possible override, eg. --I-know-what-I-am-doing
option.