Re: [PATCH 3/4] kvm tools: Add debug feature to test the IO thread

[Ingo Molnar <mingo@elte.hu>]

* Sasha Levin <levinsasha928@gmail.com> wrote:

> On Tue, 2011-04-19 at 19:52 +0300, Pekka Enberg wrote:
> > On Mon, Apr 18, 2011 at 4:02 PM, Sasha Levin <levinsasha928@gmail.com> wrote:
> > > Add --debug-io-delay-cycles and --debug-io-delay-amount to delay the completion of IO requests within virtio-blk.
> > > This feature allows to verify and debug the threading within virtio-blk.
> > >
> > > Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> > 
> > Well, to be honest, I'm not convinced we need both of these. Isn't
> > --debug-io-delay=<msec> enough for our use?
> 
> This came up during our testing.
> 
> Ingo suggested a large delay so we could easily see the results of
> threading. The problem we encountered was that having a delay right from
> the beginning will make the guest kernel take a rather long time to boot
> and would make actually testing the threading impossible.
> 
> I've added a delay before the activation of the I/O request completion
> delay to give the tester/debugger enough time to boot into the guest and
> prepare anything needed for the test.
> 
> Making it a constant is also rather hard because different kernels can
> take a very different amount of I/O requests to boot. Take the simple
> example of a whether fsck was running during the boot or not.

I suspect we'll eventually want to have some sort of 'kvm set' subcommand that 
can modify attributes of running instances? Setting the debug delay would be 
one of the options:

	kvm set MyInstance-1 --debug-io-delay-ms 10000

That way the delay can be activated in a suitable moment - and disabled again 
after testing:

	kvm set MyInstance-1 --debug-io-delay-ms 0

?

Thanks,

	Ingo