Re: [PATCH RFC] btrfs: pick device with lowest devt for show_devname

[David Sterba <dsterba@suse.cz>]

On Wed, Nov 29, 2023 at 09:54:02PM +0100, Goffredo Baroncelli wrote:
> On 29/11/2023 00.28, Anand Jain wrote:
> > 
> > 
> > On 28/11/2023 16:00, Goffredo Baroncelli wrote:
> >> On 27/11/2023 12.48, Anand Jain wrote:
> >>>
> >>>
> >>> On 11/25/23 09:09, Anand Jain wrote:
> >> [...]
> >>>> I am skeptical about whether we have a strong case to create a single
> >>>> pseudo device per multi-device Btrfs filesystem, such as, for example
> >>>> '/dev/btrfs/<fsid>-<random>/rootid=5' and which means pseudo device
> >>>> will carry the btrfs-magic and the actual blk devices something else.
> >>>>
> >>>> OR for now, regarding the umount issue mentioned above, we just can
> >>>> document it for the users to be aware of.
> >>>>
> >>>> Any feedback is greatly appreciated.
> >>>>
> >>>
> >>> How about if we display the devices list in the options, so that
> >>> user-land libs have something in the mount-table that tells all
> >>> the devices part of the fsid?
> >>>
> >>> For example:
> >>> $ cat /proc/self/mounts | grep btrfs
> >>>
> >>> /dev/sda1 /btrfs btrfs rw,relatime,space_cache=v2,subvolid=5,subvol=/,device=/dev/sda2,device=/dev/sdb3  0 0
> >>>
> >>
> >> When I developed code to find a btrfs mount point from a disk, I had to
> >> consider all the devices involved and check if one is in /proc/self/mounts.
> >>
> >> Putting the devices list as device=<xxx>,device=<yyy> doesn't change anything because
> >> the code has to manage a btrfs filesistem as "special" in any case.
> >> To get the map <btrfs-uuid> <-> <devices-list> I used libblkid.
> [...]
> > 
> > Regarding libblkid for Btrfs device discovery, I m little confused what are you referring to, an example would be helpful.
> 
> I developed a little utility to build for each btrfs filesystem:
> - all the devices involved
> - all the mountpoint (if any) where the filesystem is mounted and the subvolume used as root.
> 
> It was nice because it got all these information only using:
> - libblkid
> - parsing /proc/self/mountinfo

I think if there's one consistent approach based on libblkid then all
the related tools and projects can use that.