Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

[Anthony Liguori]

On 05/11/2010 07:15 AM, Paul Brook wrote:
> > > What about another cache=... value instead of adding more options? I'm
> > > quite sure you'll only ever need this with writeback caching. So we
> > > could have cache=none|writethrough|writeback|wb-noflush or something
> > > like that.
> > >        
> I think this table is misleading, and from a user perspective there are only 4
> interesting combinations:
>    

I agree that splitting cache would be unfortunate for users.

> cache=none:
>    No host caching. Reads and writes both go directly to underlying storage.
> Useful to avoid double-caching.
>
> cache=writethrough
>    Reads are cached. Writes go directly to underlying storage.  Useful for
> broken guests that aren't aware of drive caches.
>
> cache=writeback
>    Reads and writes are cached. Guest flushes are honoured. Note that this may
> include a guest visible knob that allows the write cache to be disabled. If
> the guest requests the cache be disabled then this should act the same as
> cache=writethrough. Closest to how real hardware works.
>
> cache=always (or a more scary name like cache=lie to defend against idiots)
>    Reads and writes are cached. Guest flushes are ignored.  Useful for dumb
> guests in non-critical environments.
>    

I really don't believe that we should support a cache=lie.  There are 
many other obtain the same results.  For instance, mount your guest 
filesystem with barrier=0.

Introducing a mode where data integrity isn't preserved means that it's 
possible for someone to accidentally use it or use it when they really 
shouldn't.  People already struggle with what caching mode they should 
use and this would just make the problem worse.

Regards,

Anthony Liguori

> IMO the other boxes in your table (disable cache but require guest flushes)
> don't make any sense.
>
> Paul
>
>