[bug-report] 5-9% FIO randomwrite ext4 perf regression on 6.12.y kernel

[Christoph Hellwig]

On Wed, Nov 20, 2024 at 06:20:12PM -0700, Jens Axboe wrote:
> There's no way that commit is involved, the test as quoted doesn't even
> touch write zeroes. Hence if there really is a regression here, then
> it's either not easily bisectable, some error was injected while
> bisecting, or the test itself is bimodal.

ext4 actually has some weird lazy init code using write zeroes.  So
if the test actually wasn't a steady state one but only run for a short
time after init, and the mentioned commit dropped the intel hack for
deallocate as write zeroes it might actually make a difference.

To check for that do a :

/sys/block/nvmeXn1/queue/write_zeroes_max_bytes

with and without that commit.