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1jdfXlVckjNnKKSrzQ9N23jG8Bs0hjt- X-Proofpoint-ORIG-GUID: 1jdfXlVckjNnKKSrzQ9N23jG8Bs0hjt- 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/-/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=,device= doesn't change > anything because > the code has to manage a btrfs filesistem as "special" in any case. > To get the map <-> I used libblkid. > > I think that a "saner" way to manage this issue, is to patch "mount" to > consider the special needing of btrfs. > > Pay attention to consider also events like, removing a device, adding a > device: > after these events how /dev/disk/by-uuid/ would be updated ? > > What about bcachefs ? Does it have the same issue ? If yes this may be > a further reason to patch "mount" instead relying to a rule (pick the > lowest devt) spread for all the projects (systemd, mount...). The display-devices-list in /proc/self/mounts method simplifies device listing, bypassing the need for sysfs or Btrfs-progs and it replaces the lowest-devt approach proposed earlier. And, yeah, all multi-device filesystems would need a special case handing in libmount. Udev's /dev/disk/by-uuid, gets updated upon an (over)write sb event. I don't know its updater, it is not in util-linux, I find no rules in /etc/udev either. Regarding libblkid for Btrfs device discovery, I m little confused what are you referring to, an example would be helpful. Thanks.