Re: [Qemu-discuss] QEMU caching modes

[Jan Schermer]

Direct IO != Synchronous IO

I'm seeing Synchronous IO when I shouldn't.

Jan

> On 20 Mar 2017, at 17:21, Alberto Garcia <address@hidden> wrote:
> 
> 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
>