Re: [Qemu-devel] [RFC PATCH] qemu-io: add drain/undrain cmd

[Fam Zheng]

On Mon, 05/15 13:26, Peter Lieven wrote:
> Am 15.05.2017 um 12:50 schrieb Fam Zheng:
> > On Mon, 05/15 12:02, Peter Lieven wrote:
> > > Hi Block developers,
> > > 
> > > I would like to add a feature to Qemu to drain all traffic from a block 
> > > so that
> > > I can take external snaphosts without the risk to that in the middle of a 
> > > write
> > > operation. Its meant for cases where where QGA freeze/thaw is not 
> > > available.
> > > 
> > > For me its enough to have this through qemu-io, but Kevin asked me to 
> > > check
> > > if its not worth to have a stable API for it and present it via QMP/HMP.
> > > 
> > > What are your thoughts?
> > For debugging purpose or a "hacky" usage where you know what you are doing, 
> > it
> > may be fine to have this. The only issue is it should be a separate flag, 
> > like
> > BlockJob.user_paused.
> 
> How can I add, remove such a flag?

Like bs->user_drained. Set it in "drain" command, then increment
bs->quiesce_counter if toggled; vise versa.

> 
> > 
> > What happens from guest perspective? In the case of virtio, the request 
> > queue is
> > not handled and -ETIMEDOUT may happen. With IDE, I/O commands are still 
> > handled,
> > the command is not effective (or rather the implementation is not complete).
> 
> That it only works with virtio is fine. However, the thing it does not work 
> correctly
> apply then also to all other users of the drained_begin/end functions, right?
> As for the timeout I only plan to drain the device for about 1 second.

It didn't matter because for IDE, the invariant (staying quiesced as long as
necessary) is already ensured by BQL.  Virtio is different because it supports
ioeventfd and data plane.

Fam