Incorrect free space calculation on RAID1 btrfs volume

[bshor]

Hello, I have a RAID1 btrfs volume running on Ubuntu 20.04 LTS. According to the btrfs fi usage command, I have approx 1.2T of free space (see also the BTRFS wiki on free space calculation). But according to duf, I only have 960Gb. The correct usage is also 1.2Tb, so the percentage should be roughly 50%. I understand the used space being ~600Gb because that is the amount of data I have, but because RAID1 doubles the data, I really have used 1.2Tb. I think it's a mistake but I understand it. But the free space left seems wrong and I don't understand.

╭───────────────────────────────────────────────────────────────────────────────────────────────────╮
│ 1 local device                                                                                    │
├─────────────────────┬──────┬────────┬────────┬───────────────────────────────┬───────┬────────────┤
│ MOUNTED ON          │ SIZE │   USED │  AVAIL │              USE%             │ TYPE  │ FILESYSTEM │
├─────────────────────┼──────┼────────┼────────┼───────────────────────────────┼───────┼────────────┤
│ /var/lib/snapd/host │ 1.8T │ 603.9G │ 961.1G │ [######..............]  33.3% │ btrfs │ /dev/sdd   │
│ fs/hdd              │      │        │        │                               │       │            │
╰─────────────────────┴──────┴────────┴────────┴───────────────────────────────┴───────┴────────────╯

Overall:
    Device size:		   3.54TiB
    Device allocated:		   1.18TiB
    Device unallocated:		   2.36TiB
    Device missing:		     0.00B
    Used:			   1.18TiB
    Free (estimated):		   1.18TiB	(min: 1.18TiB)
    Data ratio:			      2.00
    Metadata ratio:		      2.00
    Global reserve:		 512.00MiB	(used: 0.00B)

Data,RAID1: Size:603.00GiB, Used:602.67GiB (99.95%)
   /dev/sdd	 116.00GiB
   /dev/sda	 422.00GiB
   /dev/sdc	 450.00GiB
   /dev/sde	 218.00GiB

Metadata,RAID1: Size:1.00GiB, Used:768.50MiB (75.05%)
   /dev/sda	   1.00GiB
   /dev/sdc	   1.00GiB

System,RAID1: Size:32.00MiB, Used:128.00KiB (0.39%)
   /dev/sdd	  32.00MiB
   /dev/sda	  32.00MiB

Unallocated:
   /dev/sdd	 480.14GiB
   /dev/sda	 508.48GiB
   /dev/sdb	 465.76GiB
   /dev/sdc	 480.51GiB
   /dev/sde	 480.64GiB

(by the way, this is mounted as /hdd, but I have to use /dev/sdd to get the free space info for some reason)

[muesli]

As it's currently impossible to retrieve the actual mountinfo from within a snap, I'm afraid there's nothing I can do. On recent Debian & Ubuntu installs you can simply apt install duf (and there are also .debs you can download from duf's release page). Should you still be able to reproduce issues with a "regular" duf install, please let me know.

[bshor]

Thank you for the response. In the time since, I found the Ubuntu package and installed that. So this is on a different machine with Ubuntu 21.10 (Impish) running duf 0.6.2-1 with a 2-drive RAID1 configuration.

I still get the incorrect free space calculation in duf. I should be getting 2Tb free, but instead it is showing 1.6Tb free.

Here is the duf output:

╭───────────────────────────────────────────────────────────────────────────────────────╮
│ 1 local device                                                                        │
├────────────┬──────┬──────┬───────┬───────────────────────────────┬───────┬────────────┤
│ MOUNTED ON │ SIZE │ USED │ AVAIL │              USE%             │ TYPE  │ FILESYSTEM │
├────────────┼──────┼──────┼───────┼───────────────────────────────┼───────┼────────────┤
│ /data      │ 3.2T │ 1.2T │  1.6T │ [#######.............]  36.5% │ btrfs │ /dev/sdb   │
╰────────────┴──────┴──────┴───────┴───────────────────────────────┴───────┴────────────╯

And here is the btrfs usage output.

Overall:
    Device size:		   6.37TiB
    Device allocated:		   2.34TiB
    Device unallocated:		   4.03TiB
    Device missing:		     0.00B
    Used:			   2.32TiB
    Free (estimated):		   2.02TiB	(min: 2.02TiB)
    Free (statfs, df):		   1.57TiB
    Data ratio:			      2.00
    Metadata ratio:		      2.00
    Global reserve:		 512.00MiB	(used: 0.00B)
    Multiple profiles:		        no

Data,RAID1: Size:1.17TiB, Used:1.16TiB (99.48%)
   /dev/sdb	   1.17TiB
   /dev/sdd	   1.17TiB

Metadata,RAID1: Size:3.00GiB, Used:2.22GiB (74.04%)
   /dev/sdb	   3.00GiB
   /dev/sdd	   3.00GiB

System,RAID1: Size:8.00MiB, Used:192.00KiB (2.34%)
   /dev/sdb	   8.00MiB
   /dev/sdd	   8.00MiB

Unallocated:
   /dev/sdb	   1.56TiB