Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support

[Anthony Liguori]

> > > >
> > > >That means we can maintain the physical size without introducing
> > > >additional fsync()s in the allocation path.  Since we're already
> > > >writing out the header anyway, the write operation is basically
> > > >free too.
> > >
> > >  I don't see how it is free.  It's an extra write.  The good news is
> > >  that it's very easy to amortize.
> >
> > We only need to update the header field on disk when we're already
> > updating the header, so it's not even an extra write operation.
>
> Why would you ever update the header, apart from relocating L1 for 
> some reason?

To update the L1/L2 tables clean bit.  That's what prevents a check in 
the normal case where you have a clean shutdown.

Regards,

Anthony Liguori