Comment 5 for bug 1806015

I confirmed that the underlying block devices (SAS, NVME) perform the same
on the 4.4 and 4.15 kernels. Roughly 170 IOP/s direct to the SAS device
and 570 IOP/s direct to the bcache infront of the SAS device. The block
scheduler has no effect, due to the use of O_DIRECT.

I can reproduce the sysbench difference in performance on 4.15 versus 4.4.
The tuning helps though, only disabling sequential_cutoff really matters as
this enables bcache to also cache reads; in general the faster reads allow
additional writes.

During testing, I believe the core issue we're seeing between 4.4 and 4.15
is around two things:

1) ext4 fs on bionic + enable metadata_csum by default on the filesystems,
which will result in additional latency and IO as the csum is calculated and
then embedded into the journal

2) fsync performance on 4.15 is measurably slower than on 4.4, even without
csum_metadata enabled