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Re: [Qemu-discuss] QEMU caching modes

From: Alberto Garcia
Subject: Re: [Qemu-discuss] QEMU caching modes
Date: Mon, 20 Mar 2017 17:21:26 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

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.


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