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

[Kevin Wolf]

Am 22.10.2010 14:58, schrieb Anthony Liguori:
> On 10/22/2010 03:29 AM, Kevin Wolf wrote:
>>> I agree.
>>>      
>> Of course, as Laurent said a while ago, there is no specification for
>> NBD, so it's hard to say what the intended semantics is.
>>
>> However, I did have a look at the nbdserver code and it looks as if it
>> implements something similar to writethrough (namely fsync after each
>> write) only if configured this way on the server side. qemu-nbd defaults
>> to writethrough, but can be configured to use cache=none. So with either
>> server qemu as a client can't tell whether the data is safe on disk or not.
>>
>> In my book this is a strong argument for refusing to open nbd
>> connections with anything but cache=unsafe.
>>    
> 
> 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.

> 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?

Kevin