linux-kernel - Re: [PATCH] virtio-blk: set QUEUE_ORDERED

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Message-ID: <4A952A5D.5040606@redhat.com>
Date:	Wed, 26 Aug 2009 15:28:13 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Christoph Hellwig <hch@....de>, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

On 08/26/2009 03:06 PM, Rusty Russell wrote:
> On Tue, 25 Aug 2009 11:46:08 pm Christoph Hellwig wrote:
>    
>> On Tue, Aug 25, 2009 at 11:41:37PM +0930, Rusty Russell wrote:
>>      
>>> On Fri, 21 Aug 2009 06:26:16 am Christoph Hellwig wrote:
>>>        
>>>> Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which
>>>> means it does not allow filesystems to use barriers.  But the typical use
>>>> case for virtio-blk is to use a backed that uses synchronous I/O
>>>>          
>>> Really?  Does qemu open with O_SYNC?
>>>
>>> I'm definitely no block expert, but this seems strange...
>>> Rusty.
>>>        
>> Qemu can open it various ways, but the only one that is fully safe
>> is O_SYNC (cache=writethrough).
>>      
> (Rusty goes away and reads the qemu man page).
>
> 	By default, if no explicit caching is specified for a qcow2 disk image,
> 	cache=writeback will be used.
>    

It's now switched to writethrough.  In any case, cache=writeback means 
"lie to the guest, we don't care about integrity".

> Are you claiming qcow2 is unusual?  I can believe snapshot is less common,
> though I use it all the time.
>
> You'd normally have to add a feature for something like this.  I don't
> think this is different.
>    

Why do we need to add a feature for this?

-- 
error compiling committee.c: too many arguments to function

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