Re: [Qemu-devel] [PATCH] Flush pending AIO on reboot and shutdown.

[Anthony Liguori <anthony@codemonkey.ws>]

Gleb Natapov wrote:
> On Wed, Aug 13, 2008 at 09:40:34PM +0300, Avi Kivity wrote:
>   
>> Gleb Natapov wrote:
>>     
>>> but what bother me is that we
>>> consciously drop user data that we can easily save. Why? Real HW tries
>>> hard to save every bit of user data and we just decided to drop it. The
>>> difference between cancel or complete a request may be corrupted or not
>>> corrupted file system after a crash.
>>>
>>>   
>>>       
>> If a guest didn't wait for an I/O to complete, it shouldn't expect it to  
>> be on disk.
>>     
> If you'll save your swiss bank account details in file and suddenly
> your guest will crash, you'll surely appreciate if qemu will save it
> for you :)
>
>   
>> Of course, we can have the IDE layer wait instead of cancelling, which  
>> will get the request onto the disk.
>>
>>     
> That is the current approach, but the wait is done for all block IO not
> just IDE.
>   

If the guest hasn't seen confirmation that the data is on disk, as far 
as the guest is concerned, it's not on disk.

If you have a journalled file system, for instance, unless it receives 
that notification, it's going to replay whatever has happened in the 
journal since the last successful write operation.  Practically 
speaking, I don't think you're saving data.

Regards,

Anthony Liguori

> --
> 			Gleb.
>
>
>