Re: What to do about subvolumes?

[Goffredo Baroncelli <kreijack@libero.it>]

On Wednesday, 01 December, 2010, Jeff Layton wrote:
> A more common use case than CIFS or samba is going to be things like
> backup programs. They commonly look at inode numbers in order to
> identify hardlinks and may be horribly confused when there files that
> have a link count >1 and inode number collisions with other files.
> 
> That probably qualifies as an "enterprise-ready" show stopper...

I hope that a backup program, uses the pair (inode,fsid) to identify if two 
file are hardlinked... otherwise a backup of two filesystem mounted can be 
quite danguerous...


>From the statfs(2) man page:
[..]
The f_fsid field
[...]
The general idea is that f_fsid contains some random stuff such that the pair 
(f_fsid,ino) uniquely determines a file.  Some operating systems use (a 
variation on) the device number, or the device number combined  with  the  
file-system  type.   Several  OSes restrict giving out the f_fsid field to the 
superuser only (and zero it for unprivileged users), because this field is 
used in the filehandle of the file system when NFS-exported, and giving it out 
is a security concern.


And the btrfs_statfs function returns a different fsid for every subvolume.

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