On Fri, May 27, 2022 at 06:27:53PM -0700, Eric Wheeler wrote:
> > I can say that the performance of tests after the write back command for
> > all devices greatly worsens the performance of direct tests on NVME
> > hardware. Below you can see this.
>
> I wonder what is going on there! I tried the same thing on my system and
> 'write through' is faster for me, too, so it would be worth investigating.
In Ceph context, it seems not unusual to disable SSD write back cache
and see much improved performance (or the other way round: see
surprisingly low performance with write back cache enabled):
https://yourcmc.ru/wiki/Ceph_performance#Drive_cache_is_slowing_you_down
Disk controllers seem to interpret FLUSH CACHE / FUA differently.
If bcache would set FUA for cache device writes while running fio
directly on the nvme device would not, that might explain the timing
difference.
Regards
Matthias
