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

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

On 06/24/2015 10:07 PM, Dr. David Alan Gilbert wrote:
> * Wen Congyang (ghostwcy@gmail.com) wrote:
>> At 2015/6/19 18:49, Stefan Hajnoczi Wrote:
>>> On Fri, Jun 19, 2015 at 08:54:56AM +0800, Wen Congyang wrote:
>>>> 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.
>>>
>>> A monitor command to hot add/remove quorum children solves this problem
>>> and could also be used in other scenarios (e.g. user decides to take a
>>> quorum child offline).
>>>
>>
>> For replication case, we always do checkpoint again and again after
>> migration. If the disk is not synced before migration, we will use disk mirgation or mirror
>> job to sync it.
> 
> Can you document the way that you use disk migration or mirror, together
> with your COLO setup?   I think it would make it easier to understand this
> restriction.
> At the moment I don't understand how you can switch from doing a disk migration
> into COLO mode - there seems to be a gap between the end of disk migration and the start
> of COLO.

In my test, I use disk migration.

After migration and before starting COLO, we will disable disk migration:
+    /* Disable block migration */
+    s->params.blk = 0;
+    s->params.shared = 0;
+    qemu_savevm_state_begin(trans, &s->params);

If the user uses mirror job, we don't cancel the mirror job now.

> 
>> So we cannot start block replication when migration is running. We need
>> that nbd
>> client is not ready when migration is running, and it is ready between
>> migration ends
>> and checkpoint begins. Using a monotir command add the nbd client will cause
>> larger
>> downtime. So if the nbd client has been added(only not connect to the nbd
>> server),
>> we can connect to nbd server automatically.
> 
> Without the disk migration or mirror, I can't see the need for the delayed bdrv_connect.
> I can see that you want to do a disconnect at failover, however you need
> to take care because if the network is broken at the point of failover
> you need to make sure nothing blocks.

disk migration is needed if the disk is not synced before migration.

Thanks
Wen Congyang

> 
> Dave
> 
>>
>> Thanks
>> Wen Congyang
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> .
>