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

[Jamie Lokier]

Alexander Graf wrote:
> 
> On 17.05.2010, at 18:26, Anthony Liguori wrote:
> 
> > On 05/17/2010 11:23 AM, Paul Brook wrote:
> >>>> I don't see a difference between the results. Apparently the barrier
> >>>> option doesn't change a thing.
> >>>>       
> >>> Ok.  I don't like it, but I can see how it's compelling.  I'd like to
> >>> see the documentation improved though.  I also think a warning printed
> >>> on stdio about the safety of the option would be appropriate.
> >>>     
> >> I disagree with this last bit.
> >> 
> >> Errors should be issued if the user did something wrong.
> >> Warnings should be issued if qemu did (or will soon do) something other 
> >> than
> >> what the user requested, or otherwise made questionable decisions on the
> >> user's behalf.
> >> 
> >> In this case we're doing exactly what the user requested. The only 
> >> plausible
> >> failure case is where a user is blindly trying options that they clearly 
> >> don't
> >> understand or read the documentation for. I have zero sympathy for 
> >> complaints
> >> like "Someone on the Internet told me to use --breakme, and broke thinks".
> >>   
> > 
> > I see it as the equivalent to the Taint bit in Linux.  I want to make it 
> > clear to users up front that if you use this option, and you have data loss 
> > issues, don't complain.
> > 
> > Just putting something in qemu-doc.texi is not enough IMHO.  Few people 
> > actually read it.
> 
> But that's why it's no default and also called "volatile". If you prefer, we 
> can call it cache=destroys_your_image.

With that semantic, a future iteration of cache=volatile could even
avoid writing to the backing file at all, if that's yet faster.  I
wonder if that would be faster.  Anyone fancy doing a hack with the
whole guest image as a big malloc inside qemu?  I don't have enough RAM :-)

-- Jamie