Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Anthony Liguori]

Avi Kivity wrote:
> Given that we don't have a zero-copy implementation yet, it is
> impossible to generate real performance data.

Which means that it's premature to suggest switching the default to 
O_DIRECT since it's not going to help right now.  It can be revisited 
once we can do zero copy but again, I think it should be driven by 
actual performance data.  My main point is that switching to O_DIRECT 
right now is only going to hurt performance for some users, and most 
likely help no one.

> Now I don't have data that demonstrates how bad these effects are, but I
> think there is sufficient arguments here to justify adding O_DIRECT.  I
> intend to recommend O_DIRECT unless I see performance data that favours
> O_DSYNC on real world scenarios that take into account bandwidth, cpu
> utilization, and memory utilization (i.e. a 1G guest on a 32G host
> running fio but not top doesn't count).
>   

So you intend on recommending something that you don't think is going to 
improve performance today and you know in certain scenarios is going to 
decrease performance?  That doesn't seem right :-)

I'm certainly open to changing the default once we get to a point where 
there's a demonstrable performance improvement from O_DIRECT but since I 
don't think it's a given that there will be, switching now seems like a 
premature optimization which has the side effect of reducing the 
performance of certain users.  That seems like a Bad Thing to me.

Regards,

Anthony Liguori