After adding disk, used space is incorrectly reported

[sfranzen (Steven)]

Please see my comment to this other thread:

Usage data on pools and shares not updated

Some of the problems in this thread are unfortunately inherent to the use of btrfs. For example, output from the system commands df and du will generally be different from that of btrfsprogs btrfs filesystem df/du. The reason is that filesystem metadata, RAID levels and subvolumes all make the concept of “free space” much more complicated, also depending on whether you look at a whole filesystem or an individual share. The developers themselves explain this better in their FAQ section. There is…