Re: [Qemu-block] [Qemu-devel] [PATCH COLO-Block v6 07/16] Add new block driver interface to connect/disconnect the remote target

[Wen Congyang]

On 06/19/2015 12:06 AM, Stefan Hajnoczi wrote:
> On Thu, Jun 18, 2015 at 10:36:39PM +0800, Wen Congyang wrote:
>> At 2015/6/18 20:55, Stefan Hajnoczi Wrote:
>>> On Thu, Jun 18, 2015 at 04:49:12PM +0800, Wen Congyang wrote:
>>>> +void bdrv_connect(BlockDriverState *bs, Error **errp)
>>>> +{
>>>> +    BlockDriver *drv = bs->drv;
>>>> +
>>>> +    if (drv && drv->bdrv_connect) {
>>>> +        drv->bdrv_connect(bs, errp);
>>>> +    } else if (bs->file) {
>>>> +        bdrv_connect(bs->file, errp);
>>>> +    } else {
>>>> +        error_setg(errp, "this feature or command is not currently 
>>>> supported");
>>>> +    }
>>>> +}
>>>> +
>>>> +void bdrv_disconnect(BlockDriverState *bs)
>>>> +{
>>>> +    BlockDriver *drv = bs->drv;
>>>> +
>>>> +    if (drv && drv->bdrv_disconnect) {
>>>> +        drv->bdrv_disconnect(bs);
>>>> +    } else if (bs->file) {
>>>> +        bdrv_disconnect(bs->file);
>>>> +    }
>>>> +}
>>>
>>> Please add doc comments describing the semantics of these commands.
>>
>> Where should it be documented? In the header file?
> 
> block.h doesn't document prototypes in the header file, please document
> the function definition in block.c.  (QEMU is not consistent here, some
> places do it the other way around.)
> 
>>> Why are these operations needed when there is already a bs->drv == NULL
>>> case which means the BDS is not ready for read/write?
>>>
>>
>> The purpos is that: don't connect to nbd server when opening a nbd client.
>> connect/disconnect
>> to nbd server when we need to do it.
>>
>> IIUC, if bs->drv is NULL, it means that the driver is ejected? Here,
>> connect/disconnect
>> means that connect/disconnect to remote target(The target may be in another
>> host).
> 
> Connect/disconnect puts something on the QEMU command-line that isn't
> ready at startup time.
> 
> How about using monitor commands to add objects when needed instead?
> 
> That is cleaner because it doesn't introduce a new state (which is only
> implemented for nbd).
> 

The problem is that, nbd client is one child of quorum, and quorum must have 
more
than one child. The nbd server is not ready until colo is running.

Any suggestion is welcome.

Thanks
Wen Congyang