On Mon, May 16, 2011 at 04:10:21PM -0500, Anthony Liguori wrote: > To further clarify: > > Today cache=none|writethrough|writeback does two things. It: > > 1) Changes the WCE flag that's visible to the guest > > 2) Determines whether the host page cache is used for doing guest I/O > > As Christoph is very correct in pointing out, we cannot change (1) at run > time because this is guest visible. You will break a guest if you do this. > > But it's still desirable to be able to change (2) at run time. Before we > can do this properly though, we need to separate out the logic for setting > (1) vs. (2). > > And ideally, we would allow (1) to be changed by the guest itself at run > time which allows for full dynamic control. This is what he's referring to > below.
Exactly. Setting/clearing the BDRV_O_NO_FLUSH also seems useful, maybe in addition to also allowing an equivalent for the writethrough modes.
