Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/26/2010 09:03 AM, Kevin Wolf wrote:
> Am 26.05.2010 15:42, schrieb Anthony Liguori:
>    
>> On 05/26/2010 03:43 AM, Kevin Wolf wrote:
>>      
>>> Am 26.05.2010 03:31, schrieb Anthony Liguori:
>>>
>>>        
>>>> On 05/25/2010 04:01 PM, Aurelien Jarno wrote:
>>>>
>>>>          
>>>>> I really think this patch can be useful, in my own case when testing
>>>>> debian-installer (I already cache=writeback). In short all that is about
>>>>> developing and testing, as opposed to run a VM in production, can
>>>>> benefit about that. This was one of the original use case of QEMU before
>>>>> KVM arrived.
>>>>>
>>>>> Unless someone can convince me not to do it, I seriously considering
>>>>> applying this patch.
>>>>>
>>>>>
>>>>>            
>>>> There really needs to be an indication in the --help output of what the
>>>> ramifications of this option are, in the very least.  It should also be
>>>> removable via a ./configure option because no sane distribution should
>>>> enable this for end users.
>>>>
>>>>          
>>> We know better what you stupid user want?
>>>        
>> What percentage of qemu users do you think have actually read qemu-doc.texi?
>>      
> As I said, put the warning in the option name like cache=unsafe or
> something even more scary and I'm all for it.
>
>    
>> It's not a stretch for someone to have heard that cache options can
>> improve performance, and then see cache=volatile in the help output, try
>> it, and then start using it because they observe a performance improvement.
>>
>> That's not being stupid.  I think it's a reasonable expectation for a
>> user to have that their data is safe.
>>      
> You seem to think that the user is too stupid to allow him to use this
> option even if he's perfectly aware what it's doing. It's a useful
> option if it's used right.
>    

No, that's not what I said.  I'm saying we need to try hard to make a 
user aware of what they're doing.

If it spit out a warning on stdio, I wouldn't think a compile option is 
needed.  Even with help output text, I'm concerned that someone is going 
to find a bad example on the internet.

cache=unsafe addresses the problem although I think it's a bit hokey.

> We need to make clear that it's dangerous when it's used in the wrong
> cases (for example by naming), but just disabling is not a solution for
> that. You don't suggest that "no sane distribution" should ship rm,
> because it's dangerous if you use it wrong, do you?
>    

You realize that quite a lot of distributions carry a patch to rm that 
prevents a user from doing rm -rf /?

Regards,

Anthony Liguori

> Kevin
>