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

[Anthony Liguori]

Paul Brook wrote:
> > But that said, I'm concerned that this is far too confusing for users.
> > I don't think anyone is relying on disk write performance when in
> > write-back mode simply because the guest already has a page cache so
> > writes are already being completed instantaneously from the
> > application's perspective.
> >     
>
> This isn't entirely true. With IDE devices you don't have command queueing, so 
> it's easy for a large write to stall subsequent reads for a relatively long 
> time.
> I'm not sure how much this effects qemu, but I've definitely seen it happening 
> on real hardware.
>   

I think that suggests we should have a cache=wb option and if people 
report slow downs with IDE, we can observe if cache=wb helps.  My 
suspicion is that it's not going to have a practical impact because as 
long as the operations are asynchronous (via DMA), then you're getting 
native-like performance.

My bigger concern is synchronous IO operations because then a guest VCPU 
is getting far less time to run and that may have a cascading effect on 
performance.

Anyway, I'll work up a new patch with cache=wb and repost.

Regards,

Anthony Liguori

> Paul
>