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From: | Peter Lieven |
Subject: | Re: [Qemu-devel] [RFC PATCH] qemu-io: add drain/undrain cmd |
Date: | Mon, 15 May 2017 13:58:15 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 |
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.
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? 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. Peter
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