How to reproduce (I tested in Fedora 36 (kernel 5.19.2) and also on rawhide 6.1):

2.  mkfs.btrfs over a partition.
3.  mount -o lazytime,noatime
4.  touch file.dat
5.  chattr +C file.dat # turns off compression, checksumming and COW
6.  fallocate -l1G file.dat
7.  # prefill the file with random data
    fio -ioengine=psync                      -name=test -bs=1M -rw=write                 -filename=file.dat
8.  fio -ioengine=psync    -sync=1 -direct=1 -name=test -bs=4k -rw=randwrite -runtime=60 -filename=file.dat  # Will show, say, 2K IOPs
9.  fio -ioengine=io_uring -sync=1 -direct=1 -name=test -bs=4k -rw=randwrite -runtime=60 -filename=file.dat  # Will show, say, 32K IOPs
10. fio -ioengine=libaio   -sync=1 -direct=1 -name=test -bs=4k -rw=randwrite -runtime=60 -filename=file.dat  # Will show, say, 32K IOPs

Steps 9 and 10 show implausible IOPs.

This does not happen on, say, Ext4 (all the methods give roughly the same IOPs).

Removing -sync=1 on all engines on Ext4 gives immediate return (as expected because everything gets merged and finally written very fast)

Adding/Removing -sync=1 with io_uring or libaio changes nothing on BTRFS (it's definitely a bug)


I consider it's a bug in BTRFS. Very important bug because BTRFS becomes default FS in Fedora server/desktop now. This bug may cause data loss. That's why I set this bug as high priority.