Re: [Qemu-devel] [Qemu-block] [PATCH COLO v3 01/14] docs: block replication's description

[Wen Congyang <wency@cn.fujitsu.com>]

On 04/23/2015 05:55 PM, Stefan Hajnoczi wrote:
> On Wed, Apr 22, 2015 at 05:28:01PM +0800, Wen Congyang wrote:
>> On 04/22/2015 05:18 PM, Stefan Hajnoczi wrote:
>>> On Tue, Apr 21, 2015 at 05:28:01PM +0200, Paolo Bonzini wrote:
>>>> On 21/04/2015 03:25, Wen Congyang wrote:
>>>>>>> Please do not introduce "<name>+colo" block drivers.  This approach is
>>>>>>> invasive and makes block replication specific to only a few block
>>>>>>> drivers, e.g. NBD or qcow2.
>>>>> NBD is used to connect to secondary qemu, so it must be used. But the primary
>>>>> qemu uses quorum, so the primary disk can be any format.
>>>>> The secondary disk is nbd target, and it can also be any format. The cache
>>>>> disk(active disk/hidden disk) is an empty disk, and it is created before run
>>>>> COLO. The cache disk format is qcow2 now. In theory, it can be ant format which
>>>>> supports backing file. But the driver should be updated to support colo mode.
>>>>>
>>>>>> A cleaner approach is a QMP command or -drive options that work for any
>>>>>> BlockDriverState.
>>>>>
>>>>> OK, I will add a new drive option to avoid use "<name>+colo".
>>>>
>>>> Actually I liked the "foo+colo" names.
>>>>
>>>> These are just internal details of the implementations and the
>>>> primary/secondary disks actually can be any format.
>>>>
>>>> Stefan, what was your worry with the +colo block drivers?
>>>
>>> Why does NBD need to know about COLO?  It should be possible to use
>>> iSCSI or other protocols too.
>>
>> Hmm, if you want to use iSCSI or other protocols, you should update the driver
>> to implement block replication's control interface.
>>
>> Currently, we only support nbd now.
> 
> I took a quick look at the NBD patches in this series, it looks like
> they are a hacky way to make quorum dynamically reconfigurable.
> 
> In other words, what you really need is a way to enable/disable a quorum
> child or even add/remove children at run-time.
> 
> NBD is not the right place to implement that.  Add APIs to quorum so
> COLO code can use them.
> 
> Or maybe I'm misinterpreting the patches, I only took a quick look...

Hmm, if we can enable/disable or add/remove a child at run-time, it is another
choice.

Thanks
Wen Congyang

> 
> Stefan
>