Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe

[Paolo Bonzini]

On 10/24/2011 09:37 AM, Kevin Wolf wrote:
> > Why? cache=unsafe is explicitly allowing to s/data/manure/ on
> > crash.
>
> It's surely expected on a host crash, but is it for a qemu crash?
> cache=unsafe was introduced to avoid fsync() costs, which it still
> does after this patch.

I think it's not about "why is it there", but rather about "what is it 
useful for".  My interpretation of it is "I do not need the image 
anymore unless the command exits cleanly": VM installations, qemu-img 
conversions, BDRV_O_SNAPSHOT (doesn't do it yet, but it could).  Even 
SIGINT and SIGTERM would be excluded from this definition, but they cost 
nothing so it's nice to include them.

> > If you do this for raw-posix, you need to do it for all protocols.
>
> rbd could use it, too, right. Any other protocol I missed?

NBD could, but it doesn't support flush yet.

In general, even if it were useful to implement this, I'm not sure this 
is the best way to implement it.  For example you could simply clear 
BDRV_O_NO_FLUSH in qcow2_open.

Paolo