OK, some updates after rather long time. The regression seems to be mostly
triggered by reaim rapidly creating, fsyncing, and deleting small files. I was
able to reproduce the regression using a simpler stress-unlink benchmark:
stress-unlink -s -c 10000 -f 22528 16 0 /mnt
Results with this benchmark are:
AVG STDDEV
mb_optimize_scan=0: 28.285800 0.156846
mb_optimize_scan=1: 30.896600 0.323324