From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44835) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dAEZ6-0003kZ-MA for qemu-devel@nongnu.org; Mon, 15 May 2017 07:53:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dAEZ5-0008FV-Sg for qemu-devel@nongnu.org; Mon, 15 May 2017 07:53:28 -0400 Date: Mon, 15 May 2017 19:53:18 +0800 From: Fam Zheng Message-ID: <20170515115318.GA7305@lemon.lan> References: <1494842563-6534-1-git-send-email-pl@kamp.de> <20170515105032.GD23262@lemon.lan> <26b2441d-e80c-eeef-fba1-67b2529e9287@kamp.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <26b2441d-e80c-eeef-fba1-67b2529e9287@kamp.de> Subject: Re: [Qemu-devel] [RFC PATCH] qemu-io: add drain/undrain cmd List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven Cc: kwolf@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, mreitz@redhat.com 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