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Re: [Qemu-devel] [PATCH v14 7/8] Implement new driver for block replicat

From: Wen Congyang
Subject: Re: [Qemu-devel] [PATCH v14 7/8] Implement new driver for block replication
Date: Mon, 1 Feb 2016 09:13:36 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

On 01/29/2016 11:46 PM, Stefan Hajnoczi wrote:
> On Fri, Jan 29, 2016 at 11:13:42AM +0800, Changlong Xie wrote:
>> On 01/28/2016 11:15 PM, Stefan Hajnoczi wrote:
>>> On Thu, Jan 28, 2016 at 09:13:24AM +0800, Wen Congyang wrote:
>>>> On 01/27/2016 10:46 PM, Stefan Hajnoczi wrote:
>>>>> On Wed, Jan 13, 2016 at 05:18:31PM +0800, Changlong Xie wrote:
>>> I'm concerned that the bdrv_drain_all() in vm_stop() can take a long
>>> time if the disk is slow/failing.  bdrv_drain_all() blocks until all
>>> in-flight I/O requests have completed.  What does the Primary do if the
>>> Secondary becomes unresponsive?
>> Actually, we knew this problem. But currently, there seems no better way to
>> resolve it. If you have any ideas?
> Is it possible to hold the checkpoint information and acknowledge the
> checkpoint right away, without waiting for bdrv_drain_all() or any
> Secondory guest activity to complete?

There is no way to know that secondary becomes unreponsive.

> I think this really means falling back to microcheckpointing until the
> Secondary guest can checkpoint.  Instead of a blocking vm_stop() we
> would prevent vcpus from running and when the last pending I/O finishes
> the Secondary could apply the last checkpoint.  This approach does not
> block QEMU (the monitor, etc).

If secondary host becomes unresponsive, it means that we cannot do 
We should do failover in this case.

Wen Congyang

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