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

[Fam Zheng <famz@redhat.com>]

On Mon, 05/15 15:01, Peter Lieven wrote:
> Am 15.05.2017 um 14:52 schrieb Fam Zheng:
> > On Mon, 05/15 14:32, Peter Lieven wrote:
> > > Am 15.05.2017 um 14:28 schrieb Fam Zheng:
> > > > On Mon, 05/15 13:58, Peter Lieven wrote:
> > > > > Am 15.05.2017 um 13:53 schrieb 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.
> > > > > Ah okay. You wouldn't use bdrv_drained_begin/end? Because in these functions
> > > > > the counter is incremented already.
> > > > You're right, calling bdrv_drained_begin() is better.
> > > > 
> > > > > 
> > > > > > > > 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.
> > > > > Okay understood. So my use of bdrv_drained_begin/end is more an abuse of
> > > > > these functions?
> > > > Sort of. But it's not unreasonable to "extend" bdrv_drained_begin/end to cover
> > > > IDE, I just haven't thought about "how".
> > > > 
> > > > > Do you have another idea how to achieve what I want? I was thinking of throttle
> > > > > the I/O to zero. It would be enough to do this for writes, reading doesn't hurt in
> > > > > my case.
> > > > Maybe add a block filter on top of the drained node, drain it when doing so,
> > > > then queue all further requests with a CoQueue until "undrain".  (It is then not
> > > > quite to "drain" but to "halt" or "pause", though.)
> > > To get the drain for free was why I was looking at this approach. If I read correctly
> > > if I keep using bdrv_drained_begin/end its too hacky to implement it in QMP?
> > I think so.
> > 
> > > If yes, would support adding it to qemu-io?
> > I'm under the impression that you are looking to a real use case, I don't think
> > I like the idea. Also, accessing the image from other processes while QEMU is
> > using it is strongly discouraged, and there is the coming image locking
> > mechanism to prevent this from happening. Why is the blockdev-snapshot command
> > not enough?
> 
> blockdev-snapshot is enough, but I still fear the case there is suddenly too much I/O
> for the live-commit. And that the whole snapshot / commit code is more senstive than
> just stopping I/O for a second or two.

In this case, the image fleecing approach may be what you need. It creates a
temporary point in time snapshot which is lightweight and disposable. Something
like:

https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg01359.html

(Ccing John who may have more up-to-date pointers)

> 
> do you have a pointer to the image locking mechanism?

It hit qemu.git master just a moment ago. See raw_check_perm.

Fam