Re: [Qemu-devel] [PATCH v4] throttle-groups: drain before detaching ThrottleState

[Alberto Garcia]

On Fri 10 Nov 2017 04:19:34 PM CET, Stefan Hajnoczi wrote:
> I/O requests hang after stop/cont commands at least since QEMU 2.10.0
> with -drive iops=100:
>
>   (guest)$ dd if=/dev/zero of=/dev/vdb oflag=direct count=1000
>   (qemu) stop
>   (qemu) cont
>   ...I/O is stuck...
>
> This happens because blk_set_aio_context() detaches the ThrottleState
> while requests may still be in flight:
>
>   if (tgm->throttle_state) {
>       throttle_group_detach_aio_context(tgm);
>       throttle_group_attach_aio_context(tgm, new_context);
>   }
>
> This patch encloses the detach/attach calls in a drained region so no
> I/O request is left hanging.  Also add assertions so we don't make the
> same mistake again in the future.

I'm wondering about the implications of this change... is it possible
now to bypass the I/O limits simply by stopping and quickly resuming the
VM? And is that a problem?

Berto