Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Anthony Liguori <anthony@codemonkey.ws>]

Jamie Lokier wrote:
> Avi Kivity wrote:
>   
>> Jamie Lokier wrote:
>>     
>>> Why would O_SYNC be better than O_DIRECT?
>>>       
>> O_SYNC is a write-through cache.  O_DIRECT is completely uncached.  Both 
>> have their uses (including in this context), so you can't say one is 
>> better than the other.
>>     
>
> Fine, but the question is: why would O_SYNC be better for
> *data integrity* than O_DIRECT?  Referring to:
>   

"cached" is not a terribly accurate term.  O_DIRECT avoids the host page 
cache but it doesn't guarantee that the disk is using write-through.  
For that, you need to use hdparm.

O_SYNC basically turns the host page cache into a write-through cache.  
In terms of data integrity, the only question that matters is whether 
you're misleading the guest into thinking data is on the disk when it 
isn't.  Both O_DIRECT and O_SYNC accomplish this.

If you just are concerned with data integrity, O_SYNC is probably better 
because you get the benefits of host caching.  O_DIRECT is really for 
circumstances where you know that using the host page cache is going to 
reduce performance.

Regards,

Anthony Liguori

> Anthony Liguori wrote:
>   
>> For people that care about data integrity, we should be using O_SYNC,
>> not O_DIRECT anyway.
>>     
>
> Could it be connected with this, from elsewhere?
>
> Rob van Nieuwkerk wrote:
>   
>> It appears that somewhere between RH kernels 2.4.18-27.7.x and 2.4.20-18.9
>> something has changed so that my application needs a O_SYNC too besides
>> the O_DIRECT to make sure that writes will be synchronous.  If I leave
>> the O_SYNC out with 2.4.20-18.9 the write will happen physically 35
>> seconds after the write()
>>     
>
> For that, O_SYNC is used in conjunction with O_DIRECT, rather than
> instead of it.
>
> -- Jamie
>
>
>
>