Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog

[Anthony Liguori <anthony@codemonkey.ws>]

On 10/22/2010 08:57 AM, Kevin Wolf wrote:
> Am 22.10.2010 15:45, schrieb Anthony Liguori:
>    
>>>> On a physical system, if you don't have a battery backed disk and you
>>>> enable the WC on your disk, then even with cache=writethrough we're unsafe.
>>>>
>>>>          
>>> I don't think that's right. O_SYNC should guarantee that the volatile
>>> disk cache is flushed.
>>>
>>>        
>> If your filesystem does the right thing which an awful lot of them don't
>> today.
>>      
> The list of really relevant filesystems is rather short, though.
>
>    
>>>> Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe.
>>>>
>>>>          
>>> Yeah, if you do something equivalent to cache=unsafe on a lower layer,
>>> then qemu can't do much about it. Maybe you can apply the same argument
>>> to NBD, even though it's unsafe by default.
>>>
>>>
>>>        
>>>> QEMU can't guarantee safety.  The underlying storage needs to be
>>>> configured correctly.  As long as we're not introducing caching within
>>>> QEMU, I don't think we should assume we're unsafe.
>>>>
>>>> Do we have any place where we can add docs on a per-block format basis?
>>>> It would be good to at least mention for each block device how the
>>>> backing storage needed to be configured for safety.
>>>>
>>>>          
>>> docs/block-protocols.txt?
>>>
>>>        
>> Maybe docs/block/<name>.txt?  Would be a good home for the qed spec too.
>>      
> I think spec and documentation for users should be kept separate. I
> thought that's the reason why docs/specs/ exists.
>
> And if you exclude specs, I'm not sure if there's a lot left to say for
> each format. Having ten files under docs/block/ which consist of two
> lines each would be ridiculous. If contrary to my expectations we
> actually do have content for it, docs/block/<name>.txt works for me as well.
>    

Okay, sounds reasonable.

Regards,

Anthony Liguori

> Kevin
>