Re: [PATCH 1/2] btrfs: Introduce the virtual_fsid feature

[David Sterba <dsterba@suse.cz>]

On Mon, May 08, 2023 at 07:50:55PM +0800, Qu Wenruo wrote:
> On 2023/5/8 19:27, Anand Jain wrote:
> > On 05/05/2023 21:38, David Sterba wrote:
> >> On Fri, May 05, 2023 at 03:21:35PM +0800, Qu Wenruo wrote:
> >>> On 2023/5/5 01:07, Guilherme G. Piccoli wrote:
> >> This is actually a good point, we can do that already. As a conterpart
> >> to 5f58d783fd7823 ("btrfs: free device in btrfs_close_devices for a
> >> single device filesystem") that drops single device from the list,
> >> single fs devices wouldn't be added to the list but some checks could be
> >> still done like superblock validation for eventual error reporting.
> >
> > Something similar occurred to me earlier. However, even for a single
> > device, we need to perform the scan because there may be an unfinished
> > replace target from a previous reboot, or a sprout Btrfs filesystem may
> > have a single seed device. If we were to make an exception for replace
> > targets and seed devices, it would only complicate the scan logic, which
> > goes against our attempt to simplify it.
> 
> If we go SINGLE_DEV compat_ro flags, then no such problem at all, we can
> easily reject any multi-dev features from such SINGLE_DEV fs.

With the scanning complications that Anand mentions the compat_ro flag
might make more sense, with all the limitations but allowing a safe use
of the duplicated UUIDs.

The flag would have to be set at mkfs time or by btrfsune on an
unmounted filesystem. Doing that on a mounted filesystem is possible too
but brings problems with updating the state of scanned device,
potentially ongoing operations like dev-replace and more.