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

From: Hugo Mills <hugo@carfax.org.uk>
To: Filipe Manana <fdmanana@kernel.org>
Cc: dsterba@suse.cz, linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 3/4] Btrfs: check if destination root is read-only for deduplication
Date: Thu, 31 Jan 2019 16:44:39 +0000	[thread overview]
Message-ID: <20190131164439.GC4461@carfax.org.uk> (raw)
In-Reply-To: <CAL3q7H7T4Nercz9ZYtq_2yjpiPVjUX8+1xuvxnqnaDsip0bmYw@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 2120 bytes --]

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.

   Hugo.

-- 
Hugo Mills             | A clear conscience. Where did you get this taste for
hugo@... carfax.org.uk | luxuries, Bernard?
http://carfax.org.uk/  |                                          Sir Humphrey
PGP: E2AB1DE4          |                                   Yes, Prime Minister

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]