Re: finding root filesystem of a subvolume?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-08-22 10:23, Hugo Mills wrote:
> On Tue, Aug 22, 2017 at 10:12:25AM -0400, Austin S. Hemmelgarn wrote:
>> On 2017-08-22 09:53, Ulli Horlacher wrote:
>>> On Tue 2017-08-22 (09:37), Austin S. Hemmelgarn wrote:
>>>
>>>>> root@fex:~# df -T /local/.backup/home
>>>>> Filesystem     Type  1K-blocks      Used Available Use% Mounted on
>>>>> -              -    1073740800 104252160 967766336  10% /local/.backup/home
>>>>
>>>> Hmm, now I'm really confused, I just checked on the Ubuntu 17.04 and
>>>> 16.04.3 VM's I have (I only run current and the most recent LTS
>>>> version), and neither of them behave like this.
>>>
>>> I have this kind of output on all of my Ubuntu hosts:
>>>
>>> root@moep:~# grep PRETTY_NAME /etc/os-release
>>> PRETTY_NAME="Ubuntu 16.04.3 LTS"
>>>
>>> root@moep:~# df -T /usb/UF/tmp/blubb
>>> Filesystem     Type 1K-blocks    Used Available Use% Mounted on
>>> -              -     12581888 3690524   7253700  34% /usb/UF/tmp/blubb
>>>
>>> root@moep:~# btrfs subvolume show /usb/UF/tmp/blubb
>>> /usb/UF/tmp/blubb
>>>          Name:                   blubb
>>>          UUID:                   ecf8c804-d4a3-9948-89fe-b0c1971c25cb
>>>          Parent UUID:            -
>>>          Received UUID:          -
>>>          Creation time:          2017-08-22 12:54:16 +0200
>>>          Subvolume ID:           262
>>>          Generation:             23
>>>          Gen at creation:        22
>>>          Parent ID:              5
>>>          Top level ID:           5
>>>          Flags:                  -
>>>          Snapshot(s):
>>>
>>> root@moep:~# dpkg -l | grep btrfs
>>> ii  btrfs-tools                                 4.4-1ubuntu1                                 amd64        Checksumming Copy on Write Filesystem utilities
>>>
>> Hmm, interesting.  Are you using qgroups by chance?
> 
>     I get this behaviour (the "- -") only if it's a non-mounted
> subvolume:
> 
> hrm@amelia:~ $ df -T .
> Filesystem     Type  1K-blocks     Used Available Use% Mounted on
> /dev/sdb1      btrfs 117220284 95271852  18611060  84% /home
> 
> hrm@amelia:~ $ sudo btrfs sub crea foo
> Create subvolume './foo'
> 
> hrm@amelia:~ $ df -T ./foo
> Filesystem     Type 1K-blocks     Used Available Use% Mounted on
> -              -    117220284 95271880  18611032  84% /home/hrm/foo
> 
> hrm@amelia:~ $ sudo mkdir foo/bar
> hrm@amelia:~ $ df -T foo/bar
> Filesystem     Type 1K-blocks     Used Available Use% Mounted on
> -              -    117220284 95271852  18611060  84% /home/hrm/foo
> 
> hrm@amelia:~ $ mkdir foo2
> 
> hrm@amelia:~ $ sudo mount /dev/sdb1 ./foo2 -o subvol=home/hrm/foo
> 
> hrm@amelia:~ $ df -T foo2
> Filesystem     Type  1K-blocks     Used Available Use% Mounted on
> /dev/sdb1      btrfs 117220284 95272384  18610528  84% /home/hrm/foo2
> 
Wait, I think I see what's up here.  I was just calling `df -T` without 
pointing at the subvolume (which correctly ignores it because it's not 
actually mounted).  It looks like this is a side effect of the (rather 
irritating) fake mount-point behavior of subvolumes.