Re: restic backup with btrfs /

[Mark Harmstone <mark@harmstone.com>]

On 30/06/2025 2.41 pm, Christoph Hellwig wrote:
> On Mon, Jun 30, 2025 at 11:07:06AM +0100, Mark Harmstone wrote:
>> On 28/06/2025 6.33 pm, Ulli Horlacher wrote:
>>> restic (https://restic.net/) is a great backup tool but has some
>>> limitations or design flaws: one is, it believes that any subvolume is on a
>>> different filesystem. This means: "restic backup --one-file-system /" will
>>> only backup the root subvolume, but no other subvolumes like /home
>>> /var/spool etc...
>>>
>>> One has to add every subvolume to the argument list. Bad if you
>>> create new subvolumes and forget to update the backup cronjob.
>>> When you later need to restore a file, there will be none...
>>
>> This is a bug in Restic that needs to be fixed, though they may not see
>> it that way.
> 
> .. because it isn't.  btrfs is the buggy one here breaking the old
> Unix tradition also (somewhat vaguely) encoded in Posix that st_dev must
> not changed except at a mount point.

I know. But if POSIX assumes that every inode has a unique combination of
st_dev and st_ino, and that both are 64 bits, that's incompatible with the
design of btrfs.