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Date: Wed, 11 Mar 2015 15:04:54 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's
 description
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To: Wen Congyang <wency@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, qemu devel <qemu-devel@nongnu.org>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Yang Hongyang <yanghy@cn.fujitsu.com>, jsnow@redhat.com, zhanghailiang <zhang.zhanghailiang@huawei.com>

On Wed, 03/11 15:01, Wen Congyang wrote:
> On 03/11/2015 02:49 PM, Fam Zheng wrote:
> > On Wed, 03/11 14:44, Wen Congyang wrote:
> >> On 03/03/2015 03:59 PM, Fam Zheng wrote:
> >>> On Tue, 03/03 15:53, Wen Congyang wrote:
> >>>> I test qcow2_make_empty()'s performance. The result shows that it may
> >>>> take about 100ms(normal sata disk). It is not acceptable for COLO. So
> >>>> I think disk buff is necessary(just use it to replace qcow2).
> >>>
> >>> Why not tmpfs or ramdisk?
> >>
> >> Another problem:
> >> After failover, secondary write request will be written in (active disk)?
> >> It is better to write request to (nbd target). Is there any feature can
> >> be reused to implement it?
> > 
> > You can use block commit or stream to move the data.
> 
> When doing failover, we can use it to move the data. After failover,
> I need an endless job to move the data.
> 

I see what you mean. After failover, does the nbd server receive more data
(i.e. do you need a buffer to stash data from the other side)? If you commit
(active disk) to (nbd target), all the writes will go to a single image.

Fam