Re: Expected Behavior

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

On Sat, Sep 01, 2012 at 01:47:52PM +0100, Jonathan Tripathy wrote:
> On 31/08/2012 13:41, Jonathan Tripathy wrote:
> >
> >
> >On 31.08.2012 13:36, Jonathan Tripathy wrote:
> >>On 31.08.2012 04:47, James Harper wrote:
> >>>>Hi Kent,
> >>>>
> >>>>I'm going to try and reproduce it myself as well. I just
> >>>>used IOMeter in a
> >>>>Windows DomU with 30 workers, each having an io depth of 256. A *very*
> >>>>heavy workload indeed, but my point was to see if I could
> >>>>break something.
> >>>>Unless the issue is specific to windows causing problems
> >>>>(NTFS or whatever),
> >>>>I'm guessing running fio with 30 jobs and an iodepth of 256
> >>>>would probably
> >>>>produce a similar load.
> >>>>
> >>>>BTW, do you have access to a Xen node for testing?
> >>>>
> >>>
> >>>Does the problem resolve itself after you shut down the windows DomU?
> >>>Or only when you reboot the whole Dom0?
> >>>
> >>
> >>Hi There,
> >>
> >>I managed to reproduce this again. I have to reboot the entire Dom0
> >>(the physical server) for it to work properly again.
> >>
> >>James, are you able to reproduce this? Kent, are there any other
> >>tests/debug output you need from me?
> >>
> >
> >BTW, I was using IOMeter's 'default' Access Specification with the
> >following modifications: 100% random, 66% read, 33% write, and a
> >2kB size. My bcache is formatted for 512bytes.
> >-- 
> >
> Kent, is there any debug output of some sort I could switch on and
> help you figure out what's going on? If needs be, I can give you
> access to my setup here where you can run these tests yourself, if
> you're not keen installing Xen on your end :)

Shell access would probably be fastest, I suppose...

One thing that comes to mind is perhaps the load from background
writeback is slowing things down. Two things you can do:

set writeback_percent to 10, that'll enable a pd controller so it's not
going full blast

benchmark it with writeback_running set to 0 - that disables background
writeback completely.