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

[Alexander Graf]

Kevin Wolf wrote:
> Am 24.08.2010 13:56, schrieb Alexander Graf:
>   
>> Kevin Wolf wrote:
>>     
>>> Am 24.08.2010 13:02, schrieb Stefan Hajnoczi:
>>>   
>>>       
>>>> On Tue, Aug 24, 2010 at 11:40 AM, Kevin Wolf <address@hidden> wrote:
>>>>     
>>>>         
>>>>> This reverts commit 8b3b720620a1137a1b794fc3ed64734236f94e06.
>>>>>
>>>>> This fix has caused severe slowdowns on recent kernels that actually do 
>>>>> flush
>>>>> when they are told so. Reverting this patch hurts correctness and means 
>>>>> that we
>>>>> could get corrupted images in case of a host crash. This means that qcow2 
>>>>> might
>>>>> not be an option for some people without this fix. On the other hand, I 
>>>>> get
>>>>> reports that the slowdown is so massive that not reverting it would mean 
>>>>> that
>>>>> people can't use it either because it just takes ages to complete stuff. 
>>>>> It
>>>>> probably can be fixed, but not in time for 0.13.0.
>>>>>
>>>>> Usually, if there's a possible tradeoff between correctness and 
>>>>> performance, I
>>>>> tend to choose correctness, but I'm not so sure in this case. I'm not 
>>>>> sure with
>>>>> reverting either, which is why I post this as an RFC only.
>>>>>
>>>>> I hope to get some more comments on how to proceed here for 0.13.
>>>>>       
>>>>>           
>>>> Sometimes an improvement has a side effect and it makes sense to hold
>>>> back the improvement until the side effect can be resolved.  The
>>>> period of time in which users could rely on qcow2 data integrity is
>>>> small to none, I feel reverting the commit makes sense.
>>>>     
>>>>         
>>> Right, that's the vague feeling I have, too.
>>>   
>>>       
>> If we don't think of qcow2 as integer format, why don't we just default
>> to cache=unsafe there then? That way you could keep all the syncs in
>> place making it stable with cache=!unsafe, but the default for users
>> would be fast albeit unsafe, which it already is.
>>     
>
> 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.

> That said, what we should do is changing the cache mode to unsafe in
> certain places in qemu-img, e.g. in convert for the destination image.
> If it fails, you'll throw it away anyway.
>   

That would be useful either way.


Alex