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