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

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/24/2011 10:17 AM, Kevin Wolf wrote:
> >  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.
>
> I think another common interpretation is: "I don't run this VM in
> production but for development. I want the VM to go faster and I can
> recreate the image in the unlikely event that power fails during my
> work. But it certainly would be nasty."

Fair enough.

> But I think that starting to make exceptions for single block drivers
> isn't a good idea anyway. If we want bdrv_flush() to write out all
> metadata internal to qemu, I think the approach with checking the flag
> in drivers calling things like fsync() is better. The common thing is to
> do the flush.

I don't know... checking BDRV_O_NO_FLUSH in the drivers rather than in 
the generic code sounds like a layering violation.  Perhaps what you're 
after is a separation of bdrv_co_flush from bdrv_{,co_,aio_}fsync?  Then 
BDRV_O_NO_FLUSH (better renamed to BDRV_O_NO_FSYNC...) would only 
inhibit the latter.

Paolo