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From:   "NeilBrown" <neilb@suse.de>
To:     "Al Viro" <viro@zeniv.linux.org.uk>
Cc:     "Josef Bacik" <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "Christoph Hellwig" <hch@infradead.org>,
        "Chuck Lever" <chuck.lever@oracle.com>, "Chris Mason" <clm@fb.com>,
        "David Sterba" <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH/RFC 00/11] expose btrfs subvols in mount table correctly
In-reply-to: <YQNEK48+GLDvOx8t@zeniv-ca.linux.org.uk>
References: <162742539595.32498.13687924366155737575.stgit@noble.brown>,
 <20210728193536.GD3152@fieldses.org>,
 <e75ccfd2-e09f-99b3-b132-3bd69f3c734c@toxicpanda.com>,
 <YQNEK48+GLDvOx8t@zeniv-ca.linux.org.uk>
Date:   Fri, 30 Jul 2021 16:08:57 +1000
Message-id: <162762533738.21659.10411397768851876517@noble.neil.brown.name>
Precedence: bulk

On Fri, 30 Jul 2021, Al Viro wrote:
> On Wed, Jul 28, 2021 at 05:30:04PM -0400, Josef Bacik wrote:
>=20
> > I don't think anybody has that many file systems.  For btrfs it's a single
> > file system.  Think of syncfs, it's going to walk through all of the super
> > blocks on the system calling ->sync_fs on each subvol superblock.  Now th=
is
> > isn't a huge deal, we could just have some flag that says "I'm not real" =
or
> > even just have anonymous superblocks that don't get added to the global
> > super_blocks list, and that would address my main pain points.
>=20
> Umm...  Aren't the snapshots read-only by definition?

No, though they can be.
subvols can be created empty, or duplicated from an existing subvol.
Any subvol can be written, using copy-on-write of course.

NeilBrown