As I understand it in btrfs case there is two problems:
1) Metadata in btrfs can use lots of space, especially when you convert from ext4. It might happen that you have gigabytes of free space reserved for metadata, so it can not be used anymore. This can be solved with rebalancing, but that can take ages, which is actually one of the reasons zfs doesn&#x27;t have bpr rewrite feature.
2) btrfs can have mixed raid levels and in that scenario calculating free space is tricky, but people still rely on common tools, that simply give some estimates in that case.
Change the way it estimates free space and you&#x27;ll have less clueless people complaining about fs running out of it, but more will say btrfs shows too little.