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Re: [Qemu-block] [Qemu-devel] [PATCH] block/mirror: fix fail to cancel w

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] block/mirror: fix fail to cancel when VM has heavy BLK IO
Date: Wed, 24 Jan 2018 13:16:39 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 01/24/2018 12:17 AM, Liang Li wrote:
> We found that when doing drive mirror to a low speed shared storage,
> if there was heavy BLK IO write workload in VM after the 'ready' event,
> drive mirror block job can't be canceled immediately, it would keep
> running until the heavy BLK IO workload stopped in the VM. This patch
> fixed this issue.

I think you are breaking semantics here.  Libvirt relies on
'block-job-cancel' after the 'ready' event to be a clean point-in-time
snapshot, but that is only possible if there is no out-of-order pending
I/O at the time the action takes place.  Breaking in the middle of the
loop, without using bdrv_drain(), risks leaving an inconsistent copy of
data in the mirror not corresponding to any point-in-time on the source.

There's ongoing work on adding async mirroring; this may be a better
solution to the issue you are seeing.


Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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