The Btrfs filesystem has had a long and sometimes turbulent history; LWN
first wrote about it in 2007. It offers
features not found in any other mainline Linux filesystem, but reliability
and performance problems have prevented its widespread adoption. There is at
least one company that is using Btrfs on a massive scale, though:
Facebook. At the 2020
Open Source Summit North America virtual event, Btrfs developer Josef
Bacik described
why and how Facebook has invested deeply in Btrfs and where the remaining
challenges are.

Source: LWN.net – [$] Btrfs at Facebook