Re: P.S. Re: Odd snapshot subvolume

[Goffredo Baroncelli <kreijack@inwind.it>]

On 14/04/2025 20.32, Nicholas D Steeves wrote:
> Of course 'subvolume show' gets subvolume UUID, 'filesystem show' gets
> filesystem UUID, and this is too complicated.  Yes, it's logical, once
> one understands btrfs, but multiple of my colleagues have looked at
> stuff like this, thrown up their arms, and exclaimed things to the
> effect of "I have more important things to think about".

:-)

> To encourage btrfs adoption I think we need to do better.  After
> considering alternatives, I wonder if there is anything simpler/better
> than
> 
>    # findmnt -n -o SOURCE /foo | cut -d[ -f1
> 
> to get the device suitable for mounting -o subvolid=5 | subvol=/ ?  Ie:
> "Just let me see everything with as little fuss as possible. Make it
> simple!", without relying on fstab.

Below a bit simpler command options set

     # findmnt -n -v -o SOURCE /foo

However I have to point out that this is not a specific BTRFS problem. See below

	ghigo@venice:/tmp/test$ mkdir t
	ghigo@venice:/tmp/test$ mkdir t2
	ghigo@venice:/tmp/test$ truncate -s 1G disk.img
	ghigo@venice:/tmp/test$ sudo losetup -f disk.img
	ghigo@venice:/tmp/test$ sudo mkfs.ext4 /dev/loop0
	ghigo@venice:/tmp/test$ sudo mount /dev/loop0 t/
	ghigo@venice:/tmp/test$ sudo touch t/a
	ghigo@venice:/tmp/test$ sudo mkdir t/b
	ghigo@venice:/tmp/test$ sudo touch t/b/c
	ghigo@venice:/tmp/test$ sudo mount -o X-mount.subdir=b /dev/loop0 t2

	ghigo@venice:/tmp/test$ ls t2/
	c

	ghigo@venice:/tmp/test$ findmnt t2/
	TARGET       SOURCE         FSTYPE OPTIONS
	/tmp/test/t2 /dev/loop0[/b] ext4   rw,relatime

	ghigo@venice:/tmp/test$ findmnt -n -o FSTYPE,SOURCE t2/
	ext4 /dev/loop0[/b]

For *any* filesystem, it is possible to mount a subdir of a filesystem as root.
BTRFS subvolume has some special properties (e.g. it is a "barrier" for the snapshot).
However the possibility to be mounted is not one of these BTRFS special properties.

If you want to know which subvolume is mounted, you have to look to the "subvol"
option in the mount command. However even a sub directory of a subvole can be mounted


	ghigo@venice:/tmp/test$ sudo mount -o X-mount.subdir=b,subvol=/subb /dev/loop1 t5
	ghigo@venice:/tmp/test$ findmnt t5
	TARGET       SOURCE              FSTYPE OPTIONS
	/tmp/test/t5 /dev/loop1[/subb/b] btrfs  rw,relatime,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/subb

This to say that event for the common case "findmnt -n -v -o SOURCE <path>" may be
overkilling, there are some corner case where it is needed. To understand the situation I suggest to use

	ghigo@venice:/tmp/test$ findmnt -o FSTYPE,FSROOT,SOURCE -v t5
	FSTYPE FSROOT  SOURCE
	btrfs  /subb/b /dev/loop


> 
> Cheers,
> Nicholas
> 

Ciao
Goffredo

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