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I'm aware of this regression and the cause is understood: the flagged commit adds a per-COW xarray store on every COW'd metadata block, and on this single-task 4k randwrite ftruncate workload that store is what costs the write_iops. A fix has been posted to linux-btrfs that replaces the xarray with a non-allocating fixed inline buffer and recovers the lost throughput: "btrfs: replace writeback inhibition xarray with a fixed inline buffer" It carries the Reported-by and Closes tags pointing at this report. Thanks, Leo