Re: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

[Kevin Wolf <kwolf@redhat.com>]

Am 24.08.2010 14:27, schrieb Avi Kivity:
>   On 08/24/2010 03:21 PM, Alexander Graf wrote:
>> Avi Kivity wrote:
>>>   On 08/24/2010 03:12 PM, Alexander Graf wrote:
>>>>> Well, safety is not boolean. Considering to make it mostly safe instead
>>>>> of completely safe because of the performance doesn't mean that we
>>>>> should make it completely unsafe.
>>>>>
>>>> What is safety then? A vague feeling of "oh today is monday so my data
>>>> is safe, but on tuesday I always lose my image data"? Either we promise
>>>> to keep data safe or we don't. There is no in between.
>>>>
>>> Do you drive a car?
>> Would you buy a car where the breaks are known to not always work? ;)
> 
> That's not the case, even with cache=unsafe.
> 
>>> Though in general I agree we shouldn't compromise on data integrity.
>> That's my point. Either we go for it or we don't.
> 
> I don't know how bad the performance regression is, and how large the 
> integrity risk is.  I'd default towards preserving integrity, but maybe 
> this situation is different.

I have reports of installations taking like 50 min instead of 14 min. My
own qemu-io based test goes up from 1 s to 23 s. And I think the winner
is Michael's image conversion which went up from 30 s to 49 min.

So it's not like we're talking about just some 10 or 20 percent.

Kevin