Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

Avi Kivity
2009-08-26T12:28:48Z

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?

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