Calculation of disk usage through du entails maintaining a set of all inodes represented by files under the tree, such that hard links will be counted only once with respect to total disk usage.
However, if separate inodes on a Btrfs system share the same underlying extents on the file system through reflinks, then du will still treat them as completely separate, counting the total disk usage as the sum of the sizes for each inode. Thus, the same extent will be counted for as many files as use it.
Is any utility available that will examine the reflinks of all files in a directory tree to compute an accurate estimate of disk usage within a Btrfs system?
btrfs fi du. Seeman 8 btrfs-filesystem. – Kamil Maciorowski Aug 23 '23 at 06:55