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
