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The question is not trivial. BTRFS is COW file system and one object stored on hard disk can be referenced by many files.

I use BTRFS snapshots as a part of a backup solution on my production server. This way I have space-efficient, browsable history of a given subvolume (I use a modification of the SnapBtr).

I have several independent btrfs subvolumes for different purposes and a backup scheme for each one. When the free space is running out, I can get the most unneeded backup snapshot from each backup pool based on the smart logarithmic-time-cost algorithm of SnapBtr.

I need a way to weight the amount of data that will be freed after I remove each backup with age of the old snapshot and importance of its backup pool. I am missing the former information.

I understand that the process of calculation of the free space on the BTRFS is neither trivial nor quick. I need something that would simulate the subvolume's deletion to get the size of the would-be freed space.

Can anyone help me? Should I post this message to the [email protected]?

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2 Answers 2

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As demonstrated here, this is actually fairly simple to do.

First, enable btrfs quotas:

# btrfs quota enable /btrfs_subvolume

And then run:

# btrfs qgroup show /btrfs_subvolume OR # btrfs qgroup show -f /btrfs_subvolume

Which in Btrfs v3.18.2 shows you this:

qgroupid rfer excl -------- ---- ---- 0/260 1.09GiB 1.09GiB

The 0/260 is the subvolume ID, and the excl is the exclusive data in the subvolume. If you delete the subvolume, that's how much space you'll free up.

Edit: According to this link, this appears to be the official recommended way to do this.

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btrfsQuota.py makes the output of btrfs qgroup show more readable, replacing subvol IDs with the names of the subvols, e.g.:

subvol group total unshared ------------------------------------------------------------------------------- test/a 0/970 20.05M 0.05M test/b 0/971 20.05M 10.05M test/c 0/972 30.05M 10.05M test/d 0/973 30.05M 0.05M test/e 0/974 30.05M 10.05M
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    It doesn't work with default btrfs-tools version of Ubuntu 14.04. You need to install a newer one, and then everything is ok Commented Jun 4, 2016 at 7:35
  • @AdamRyczkowski I'm using the Python 3 version of that script now, and it works fine. Commented Aug 3, 2017 at 18:44

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