On Mon, Jul 29, 2024 at 09:40:57PM +1000, Aleksa Sarai wrote: > On 2024-07-29, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote: > > On Mon, Jul 29, 2024 at 12:57 PM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > > > > On Mon, Jul 29, 2024 at 12:40:35PM +0200, Florian Weimer wrote: > > > >> > On Mon, Jul 29, 2024 at 08:55:46AM +0200, Florian Weimer wrote: > > > >> >> It was pointed out to me that inode numbers on Linux are no longer > > > >> >> expected to be unique per file system, even for local file systems. > > > >> > > > > >> > I don't know if I'm parsing this correctly. > > > >> > > > > >> > Are you claiming on-disk inode numbers are not guaranteed unique per > > > >> > filesystem? It sounds like utter breakage, with capital 'f'. > > > >> > > > >> Yes, POSIX semantics and traditional Linux semantics for POSIX-like > > > >> local file systems are different. > > > > > > > > Can you link me some threads about this? > > > > > > Sorry, it was an internal thread. It's supposed to be common knowledge > > > among Linux file system developers. Aleksa referenced LSF/MM > > > discussions. > > > > So much for open development :-P > > To be clear, this wasn't _decided_ at LSF/MM, it was brought up as a > topic. There is an LWN article about the session that mentions the > issue[1]. A discussion about inode numbers or subvolumes comes up every year with better of worse suggestions what to do about it. > My understanding is that the btrfs and bcachefs folks independently > determined they cannot provide this guarantee. As far as I understand, > the reason why is that inode number allocation on btree filesystems > stores information about location and some other bits (maybe subvolumes) > in the bits, making it harder to guarantee there will be no collisions. No, on btrfs the inode numbers don't encode anything about location, it's a simple number. The inode numbers remain the same when a snapshot is taken as it's a 1:1 clone of the file hierarchy, the directory representing a subvolume/snapshot has fixed inode number 256. The only difference is the internal subvolume id.
