Re: [Qemu-devel] [PATCH v5 2/4] block: add the block queue support

[Zhi Yong Wu]

On Fri, Aug 12, 2011 at 4:42 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Aug 12, 2011 at 9:10 AM, Zhi Yong Wu <address@hidden> wrote:
>> On Tue, Aug 9, 2011 at 8:49 PM, Stefan Hajnoczi <address@hidden> wrote:
>>> On Tue, Aug 9, 2011 at 5:17 AM, Zhi Yong Wu <address@hidden> wrote:
>>>> +/* The APIs for block request queue on qemu block layer.
>>>> + */
>>>> +
>>>> +static void qemu_block_queue_cancel(BlockDriverAIOCB *acb)
>>>> +{
>>>> +    qemu_aio_release(acb);
>>>> +}
>>>> +
>>>> +static AIOPool block_queue_pool = {
>>>> +    .aiocb_size         = sizeof(struct BlockDriverAIOCB),
>>>> +    .cancel             = qemu_block_queue_cancel,
>>>> +};
>>>
>>> The lifecycle of block_queue_pool acbs should be like this:
>>>
>>> When a request is queued we need a BlockQueue acb because we have no
>>> real acb yet.  So we get an acb from block_queue_pool.
>>>
>>> If the acb is cancelled before being dispatched we need to release the
>>> acb and remove the request from the queue.  (This patch does not
>>> remove the request from the queue on cancel.)
>>>
>>> When the acb is dispatched we need to keep track of the real acb (e.g.
>>> from qcow2).  The caller will only ever see our acb because there is
>>> no way to give them the pointer to the new acb after dispatch.  That
>>> means we need to keep the our acb alive for the entire lifetime of the
>>> request.  (This patch currently releases our acb when the request is
>>> dispatched.)
>>>
>>> If the acb is cancelled after being dispatched we need to first cancel
>>> the real acb and then release our acb.
>>>
>>> When the acb is dispatched we need to pass qemu_block_queue_callback
>>> as the cb function to handler.  Inside qemu_block_queue_callback we
>> When one block request have been reenqueued multiple times, it will be
>> difficult to store original cb function.
>
> The challenge is that blkqueue creates an acb and returns it to the
> user when a request is enqueued.  Because the user now has the acb we
> must keep this acb around for the lifetime of the request.
>
> The current code will create a new acb if the request is enqueued
> again.  We should avoid this.
>
> The code needs to be structured so that a queued acb can be dispatched
> directly - without creating a new BlockQueueAIOCB.
Maybe we can workaround this issue by temp stashing our acb to one
field of block queue.
pls see my public git tree.:)

>
> Stefan
>



-- 
Regards,

Zhi Yong Wu