Re: [Qemu-discuss] QEMU caching modes

[Alberto Garcia]

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