Re: Expected Behavior

[Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Thu, Aug 30, 2012 at 01:18:54PM +0100, Jonathan Tripathy wrote:
> 
> On 30.08.2012 08:21, Jonathan Tripathy wrote:
> >On 30/08/2012 08:15, Jonathan Tripathy wrote:
> >>Hi There,
> >>
> >>On my WIndows DomU (Xen VM) which is running on a LV which is
> >>using bcache (against two SSD in MDRAID1 and a MD-RAID10 spindle
> >>array), I ran an IOMeter test for about 2 hours (with 30 workers
> >>and a io depth of 256). This was a very heavy workload (Got an
> >>average iops of about 6.5k). After I stopped the test, I then
> >>went back to fio on my Linux Xen Host (Dom0). The random write
> >>performance isn't as good as it was before I started the IOMeter
> >>test. It used to be about 25k and now showed about 7k iops. I
> >>assumed that maybe this was due to the fact that bcache was
> >>writing out dirty data to the spindles so the SSD was busy.
> >>
> >>However, this morning, after the spindles have calmed down,
> >>performance of fio is still not great (still about 7k).
> >>
> >>Is there something wrong here? What is expected behavior?
> >>
> >>Thanks
> >>
> >BTW, I can confirm that this isn't an SSD issue, as I have a
> >partition on the SSD that I kept seperate from bcache and I'm getting
> >excellent (about 28k) iops performance there.
> >
> >It's as if after the heavy workload I did with IOMeter, bcache has
> >somehow throttled the writeback cache?
> >
> >Any help is appreciated.
> >
> 
> I'd like to add that a reboot pretty much solves the issue. This
> leads me to believe that there is a bug in the bcache code that
> causes performance to drop the more it gets used.
> 
> Any ideas?

Weird!

Yeah, that definitely sounds like a bug. I'm going to have to try and
reproduce it and go hunting. Can you think of anything that might help
with reproducing it?