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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <4C8CD0DA.40401@codemonkey.ws> (Anthony Liguori's message of
	"Sun, 12 Sep 2010 08:08:42 -0500")
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Date: Wed, 15 Sep 2010 18:10:48 +0200
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Subject: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the
	guest	starts
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Avi Kivity <avi@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

Anthony Liguori <anthony@codemonkey.ws> wrote:
> On 09/12/2010 05:42 AM, Avi Kivity wrote:
>>  On 09/11/2010 05:04 PM, Anthony Liguori wrote:
>>> This fixes a couple nasty problems relating to live migration.
>>>
>>> 1) When dealing with shared storage with weak coherence (i.e. NFS),
>>> even if
>>>     we re-read, we may end up with undesired caching.  By delaying
>>> any reads
>>>     until we absolutely have to, we decrease the likelihood of any
>>> undesirable
>>>     caching.
>>>
>>> 2) When dealing with copy-on-read, the local storage acts as a
>>> cache.  We need
>>>     to make sure to avoid any reads to avoid polluting the local cache.
>>>
>>> +
>>>   static void ide_identify(IDEState *s)
>>>   {
>>>       uint16_t *p;
>>> @@ -105,6 +132,8 @@ static void ide_identify(IDEState *s)
>>>       return;
>>>       }
>>>
>>> +    guess_geometry(s);
>>> +
>>
>> This can cause a disk read, no?  Shouldn't it be made asynchronous?
>
> Yes, it should.  I'm not sure there's an easy way to make it
> asynchronous though not because of the block layer but because of how
> these functions are called.
>
>>
>> Or just move it to just before the guest starts?
>
> We don't really have a notion of "guest starts" today although maybe
> we should.

I agree that we should have more states.  Expecting Migration/Migration
done are quite special casses that are handled adhoc now.  Having
some state like: NeverRun will make trivial to have vm_start() do the
right thing on the 1st run.

Later, Juan.

> Regards,
>
> Anthony Liguori