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Date: Mon, 20 Oct 2014 23:09:30 +0800
From: Zhang Haoyu <ahzhanghaoyu@gmail.com>
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Subject: Re: [Qemu-devel] [question] savevm/delvm: Is it necessary to
 perform bdrv_drain_all before savevm and delvm?
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To: Kevin Wolf <kwolf@redhat.com>, Zhang Haoyu <zhanghy@sangfor.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Max Reitz <mreitz@redhat.com>

 >> Hi,
 >>
 >> I noticed that bdrv_drain_all is performed in load_vmstate before 
bdrv_snapshot_goto,
 >> and bdrv_drain_all is performed in qmp_transaction before 
internal_snapshot_prepare,
 >> so is it also neccesary to perform bdrv_drain_all in savevm and delvm?
 >
 >Definitely yes for savevm. do_savevm() calls it indirectly via
 >vm_stop(), so that part looks okay.
 >
Yes, you are right.

 >delvm doesn't affect the currently running VM, and therefore doesn't
 >interfere with guest requests that are in flight. So I think that a
 >bdrv_drain_all() isn't needed there.
 >
I'm worried about that there are still pending IOs while deleting snapshot,
then is it possible that there is concurrency problem between the 
process of deleting snapshot
and the coroutine of io read/write(bdrv_co_do_rw) invoked by the pending 
IOs?
This coroutine is also in main thread.
Am I missing something?

Thanks,
Zhang Haoyu

 >Kevin