On Fri, Mar 17, 2017 at 09:52:27PM +0100, Jan Schermer wrote:
> I'm trying to understand when/what IO is synchronous and why, and so
> far what I'm seeing I can only describe as "weird".
>
> With cache=none, all IO should pass from the QEMU process to the device
> as-is. Right?
Here's the summary of what the 'cache' option does in practice:
cache=none -> direct I/O (the host page cache is bypassed).
See O_DIRECT in open(2)
cache=directsync -> direct I/O + flush after each write
cache=writethrough -> flush after each write
cache=unsafe -> guests flushes are ignored
cache=writeback -> no special options (this is the default)
> If so, that's not what I see - I see (with btrace) write IO as
> asynchronous (W) in the guest, but synchronous (WS) on the host.
Looks like what you're seeing is the direct I/O.
Berto
