Re: [Qemu-devel] [Patch v12 resend 00/10] Block replication for continuous checkpoints

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

On 12/23/2015 06:04 PM, Stefan Hajnoczi wrote:
> On Thu, Dec 17, 2015 at 02:22:14PM +0800, Wen Congyang wrote:
>> Stefan:Ping...
>>
>> What about this feature? I have worked for it about 1 year, but it is still in the
>> way...
> 
> The code still has TODOs.  What is the plan for supporting replication
> after failover?  This feature seems critical because anyone who wants FT
> won't be able to use this code unless it supports FT after the first
> failure.

We have implemented it based on an old version qemu. To keep the logical
simple, we don't post them now. We will post them after this feature is merged
into qemu.

> 
> ---
> 
> Adding new block layer APIs that are replication-specific is not clean.
> Only the replication block driver cares about the start/stop/checkpoint
> interface.
> 
> It is cleaner to have a separate API and data structure for block
> replication.
> 
> The replication code should define its own BlockReplicationOps struct
> and allow objects to register themselves.  Then it's no longer necessary
> to modify the core block layer to forward start/stop/checkpoint calls.
> 
> Something like:
> 
> typedef struct BlockReplicationOps BlockReplicationOps;
> typedef struct BlockReplicationState {
>     const BlockReplicationOps *ops;
>     QLIST_ENTRY(BlockReplicationState) list;
> } BlockReplicationState;
> 
> typedef struct {
>     void start(BlockReplicationState *brs, Error **errp);
>     void stop(BlockReplicationState *brs, Error **errp);
>     void checkpoint(BlockReplicationState *brs, Error **errp);
> } BlockReplicationOps;
> 
> static QLIST_HEAD(BlockReplicationState) block_replication_states;
> 
> void block_replication_add(BlockReplicationState *brs);
> void block_replication_remove(BlockReplicationState *brs);
> 
> The replication block driver would add/remove itself.  The quorum block
> driver probably doesn't need to be modified (I think in your current
> patches you modify it just to forward the start/stop/checkpoint calls to
> a particular quorum child).

Yes, it is the major purpose. We also do some check in the quorum driver: 
we don't allow more than one child support block replication.

Thanks
Wen Congyang

> 
> Stefan
>