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Re: [Qemu-discuss] QEMU caching modes

From: Narcis Garcia
Subject: Re: [Qemu-discuss] QEMU caching modes
Date: Mon, 20 Mar 2017 22:01:49 +0100

When cache=unsafe , is the data flushed as configured in host's Linux

El 20/03/17 a les 17:21, Alberto Garcia ha escrit:
> 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

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