Re: [Qemu-devel] [RFC PATCH v2 2/4] Add block-queue

[Stefan Hajnoczi]

On Wed, Nov 17, 2010 at 1:41 PM, Kevin Wolf <address@hidden> wrote:
> Am 17.11.2010 13:43, schrieb Stefan Hajnoczi:
>>> A typical sequence in qcow2 (simple cluster allocation) looks like this:
>>>
>>> 1. Update refcount table
>>> 2. bdrv_flush
>>> 3. Update L2 entry
>>>
>>> If we delay the operation and get three of these sequences queued before
>>> actually executing, we end up with the following result, saving two syncs:
>>>
>>> 1. Update refcount table (req 1)
>>> 2. Update refcount table (req 2)
>>> 3. Update refcount table (req 3)
>>> 4. bdrv_flush
>>> 5. Update L2 entry (req 1)
>>> 6. Update L2 entry (req 2)
>>> 7. Update L2 entry (req 3)
>>
>> How does block-queue group writes 1-3 and 5-7 together?  I thought
>> reqs 1-3 will each have their own context but from a quick look at the
>> code I don't think that is the case for qcow2 in patch 4.
>>
>> Another way of asking is, how does block-queue know that it is safe to
>> put writes 1-3 together before a bdrv_flush?  Why doesn't it also put
>> writes 5-7 before the flush?
>>
>> Perhaps your current qcow2 implementation isn't taking advantage of
>> block-queue bdrv_flush() batching for concurrent write requests?
>>
>> I'm missing something ;).
>
> Yes, you are. ;-)
>
> The contexts are indeed the mechanism that it uses to achieve this. Have
> a look at qcow_aio_setup, patch 4/4 adds a blkqueue_init_context() call
> there.

But there is a single context per qcow2 state.  So this works for
sequential requests but I guess the concurrent aio requests case
doesn't work?

Stefan