Am 27.03.2015 um 18:12 hat Eric Blake geschrieben:
> On 03/27/2015 09:36 AM, Michael Tokarev wrote:
> > Wonder how to specify cache mode, or should I open these with proper
> > O_DIRECT/O_SYNC/whatever?  It looks like it's possible to change O_DIRECT
> > at runtime but not O_SYNC.
> > 
> > And the more interesting question is how to do that from shell.
> 
> Redirections only get you so far in shell; you may need a wrapper C
> program go get O_DIRECT and/or O_SYNC pre-set.  Then again, if you use
> QMP and pass over the Unix socket, you need a C program anyways.

O_DIRECT can be set with fcntl(), so qemu takes care of that. O_SYNC
is completely unused on Linux these days, so that shouldn't be a problem
either. (Other platforms use it as a misguided attempt of approximating
O_DIRECT. We should really error out instead.)

So if I'm not mistaken, just having one read-only and one read-write fd
should be enough for any configuration in practice.

Kevin