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So add a sysfs >>> interface devinfo//fsid to show the fsid of the device. >>> >>> For example: >>>   $ cd /sys/fs/btrfs/b10b02a5-f9de-4276-b9e8-2bfd09a578a8 >>> >>>   $ cat devinfo/1/fsid >>>   c44d771f-639d-4df3-99ec-5bc7ad2af93b >>>   $ cat  devinfo/3/fsid >>>   b10b02a5-f9de-4276-b9e8-2bfd09a578a8 >> >> How do you create such setup? I can't reproduce it. >> >> The simplest seeding: >> >>    mkfs.btrfs /dev/sdx >>    btrfstune -S 1 /dev/sdx >>    mount /dev/sdx mnt >>    ... the device has the same FSID as is the sysfs directory name >> >> With a new device and removed the seeding one: >> >>    btrfs device add /dev/sdy mnt > > At this step, we generate a new fsid for the writeable FS. Let's call > it sprout-fsid. (If you check the sys-fs fsid, you will have two fsid > here). > > Also, at this step, > the >     fs_info->fs_devices->fsid > changes from seed-fsid to the sprout-fsid. > So, we should make the also a writeable without the need to call > 'remount,rw' explicitly IMO. What do you think? > More importantly, the fs_info->super_copy is of sprout device. So there isn't any reason that we should maintain the new sprout fs as readonly after the device add. >>    mount -o remount,rw mnt >>    btrfs device delete /dev/sdx mnt >>    ... both devices have the same fsid as well > > > Thanks, Anand > > > >