Re: Testing if two open descriptors refer to the same inode

[Christoph Hellwig]

On Tue, Jul 30, 2024 at 12:31:57PM +1000, Dave Chinner wrote:
> There are at least two different "is this inode identical"
> use cases that {st_dev,st_ino} is being used for.
> 
> The first, as Florian described, is to determine if two open fds
> refer to the same inode for collision avoidance.
> 
> This works on traditional filesystems like ext4 and XFS, but isn't
> reliable on filesystems with integrated snapshot/subvolume
> functionality.

It's not about snapshot, it's about file systems being broken.  Even
btrfs for example always has a unique st_dev,st_ino pair, it can
just unexpectly change at any subvolume root and not just at a mount
point.

> That is our long term challenge: replacing the use of {dev,ino} for
> data uniqueness disambiguation. Making the identification of owners
> of non-unique/shared data simple for applications to use and fast
> for filesystems to resolve will be a challenge.

I don't think there is any way to provide such a guarantee as there
is so many levels of cloning or dedup, many of which are totally
invisible to the high level file system interface.