[linux-lvm] writeback cache not promoting writes

[Max Power]

Hi,

Am 12.01.2015 um 01:35 schrieb Linda A. Walsh:
>    I don't know how it interacts in the case you have setup, but
> oflag=direct usually has 'dd' writing to the physical device -- not any
> system
> cache.  I don't know if lvm's cache changes that behavior, but you might
> try it w/o the direct flag?

I tried it without the flag but now its even worse. I got ~33MB/s (slow
drive normally performs 100MB/s and fast drive ~450MB/s).

I understand that 'dd' is not the normal use scenario of a disk. But why
does the caching (with the tunables I set) is making a difference here?
I would expect that with x_threshold and x_adjustment set to 0 there is
no difference between normal fs use and dd.

I read this written by Joe Thornber in this mailing list a few days ago:
> In writeback mode:
>
>    if block is on ssd
>       write to ssd, complete bio once written
>       increment write hit counter
>    else
>       write to origin and complete
>       increment write miss counter

This would explain why I do not see any write hits. I only got
increasing write misses by writing to the drive (even when writing again
to the same blocks). Is there an option to influence the algorithm?