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Date: Tue, 14 Sep 2010 12:08:37 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] qcow2 performance plan
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>

On 09/14/2010 11:28 AM, Avi Kivity wrote:
>  On 09/14/2010 06:16 PM, Anthony Liguori wrote:
>>
>> Right, it should only freeze if the L2 table needs to be allocated, 
>> not if it only needs to be updated.  IOW,
>>
>> diff --git a/block/qed.c b/block/qed.c
>> index 4c4e7a2..0357c03 100644
>> --- a/block/qed.c
>> +++ b/block/qed.c
>> @@ -948,7 +948,7 @@ static void qed_aio_write_data(void *opaque, int 
>> ret,
>>      }
>>
>>      /* Freeze this request if another allocating write is in 
>> progress */
>> -    if (need_alloc) {
>> +    if (ret == QED_CLUSTER_L1) {
>>          if (acb != QSIMPLEQ_FIRST(&s->allocating_write_reqs)) {
>>              QSIMPLEQ_INSERT_TAIL(&s->allocating_write_reqs, acb, next);
>>          }
>>
>> It's being a bit more conservative than it needs to be.
>
> Yes, I hit this too.  So without this patch, it does serialize all 
> allocating writes?

Yes, but my patch is not enough as it turns out.

When dealing with O_DIRECT, we have to handle RMW on our own which means 
we need to serialize access to the same sector.

The way we're planning on addressing this in the short term is to break 
the single allocator queue into a per-L2 table queue.  So writes to the 
same L2 would be serialized but writes to different L2s would not be 
serialized.

Regards,

Anthony Liguori

> If multiple requests need to update pointers in L2, will those updates 
> generate one write per request, or just two writes (one write from the 
> first request, another from all those that serialized after it)?
>