[dm-devel] Significantly dropped dm-cache performance in 4.13 compared to 4.11

[Stefan Ring]

On Tue, Nov 7, 2017 at 3:41 PM, Joe Thornber <thornber at redhat.com> wrote:
> On Fri, Nov 03, 2017 at 07:50:23PM +0100, Stefan Ring wrote:
>> It strikes me as odd that the amount read from the spinning disk is
>> actually more than what comes out of the combined device in the end.
>
> This suggests dm-cache is trying to promote too way too much.
> I'll try and reproduce the issue, your setup sounds pretty straight forward.

I think it's actually the most straight-forward you can get ;).

I've also tested kernel 4.12 in the meantime, which behaves just like
4.13. So the difference in behavior seems to have been introduced
somewhere between 4.11 and 4.12.

I've also done plain dd from the dm-cache disk to /dev/null a few
times, which wrote enormous amounts of data to the SDD. My poor SSD
has received the same amount of writes during the last week that it
has had to endure during the entire previous year.