Hi, On 7/17/19 1:24 AM, Ulli Horlacher wrote: > > I thought, I can recognize a snapshot when it has a Parent UUID, but this > is not true for snapshots of toplevel subvolumes: > > root@trulla:/# btrfs version > btrfs-progs v4.5.3+20160729 > > root@trulla:/# btrfs subvolume show /mnt/tmp > /mnt/tmp is toplevel subvolume > > root@trulla:/# btrfs subvolume snapshot /mnt/tmp /mnt/tmp/ss > Create a snapshot of '/mnt/tmp' in '/mnt/tmp/ss' > > root@trulla:/# btrfs subvolume create /mnt/tmp/xx > Create subvolume '/mnt/tmp/xx' > > root@trulla:/# btrfs subvolume show /mnt/tmp/ss > /mnt/tmp/ss > Name: ss > UUID: 7732bdde-0485-204e-b41b-833376e791da > Parent UUID: - > Received UUID: - > Creation time: 2019-07-17 01:02:48 +0200 > Subvolume ID: 270 > Generation: 60 > Gen at creation: 60 > Parent ID: 5 > Top level ID: 5 > Flags: - > Snapshot(s): > > root@trulla:/# btrfs subvolume show /mnt/tmp/xx > /mnt/tmp/xx > Name: xx > UUID: 342b2065-1679-8245-bd76-8da598cc33d8 > Parent UUID: - > Received UUID: - > Creation time: 2019-07-17 01:03:02 +0200 > Subvolume ID: 271 > Generation: 61 > Gen at creation: 61 > Parent ID: 5 > Top level ID: 5 > Flags: - > Snapshot(s): > > How do I know that /mnt/tmp/ss is a snapshot? > I cannot see a snapshot identifier. > The technical answer is: * The uuid of the top-level subvol (id 5) is 00000000-0000-0000-0000-000000000000 * The parent uuid of your snapshot/clone is also 0000... as a result. * For interpretation of parent uuid field, there is no distinction between "has no parent uuid" (the field is empty, which technically means all bits are 0) and the actual value 0000... * So... the tooling displays it as "-". ¯\_(ツ)_/¯ The practical answer is: * Don't put anything in the top level subvol that would make you want to snapshot it. Note: In btrfs design, there is no technical difference between a snapshot and a writable clone (as opposed to e.g. netapp snapshots and clones). This makes the design less complicated, but it results in a collection of known usability issues, like that if you expect them to be different, they still aren't. Hans
