Making rounds this week was word of a "500~2000% performance regression" knocking Btrfs on Linux 5.10. For a simple test case like extracting a large .tar.zst file could go from taking just around 15 seconds to nearly five minutes or in other cases like from 5 seconds to over 30 seconds. The regression was bisected to a fundamental Btrfs change in Linux 5.10 and reproduced on bare metal while running Btrfs within a virtual machine didn't trigger the major slowdown.
Josef Bacik was able to get to the bottom of it on Christmas Eve. He noted , " Alright to close the loop with this, this slipped through the cracks because I was doing a lot of performance related work, and specifically had been testing with these patches on top of everything. These patches bring the performance up to around 40% higher than baseline. In the meantime we'll probably push this partial revert into 5.10 stable so performance isn't sucking in the meantime. Thanks. "
So Linux 5.10 may still be regressing but after the holidays will likely see a partial revert on the behavior so the LTS kernel isn't in such bad shape. The patches he was referencing are in regards to improving preemptive ENOSPC [error no space left on disk] flushing.
Rene Rebe who reported the regression commented in the end, " I indeed tested the linux-btrfs for-5.11 and found the performance some 50% better. I would hope that can be brought back to 5.9 levels sometime soon ;-) "