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

[Anthony Liguori]

Mark Wagner wrote:
> If you stopped and listened to yourself, you'd see that you are making 
> my point...
>
> AFAIK, QEMU is neither designed nor intended to be an Enterprise 
> Storage Array,
> I thought this group is designing a virtualization layer.  However, 
> the persistent
> argument is that since Enterprise Storage products will often 
> acknowledge a write
> before the data is actually on the disk, its OK for QEMU to do the same.

I think you're a little lost in this thread.  We're going to have QEMU 
only acknowledge writes when they complete.  I've already sent out a 
patch.  Just waiting a couple days to let everyone give their input.

> If QEMU
> had a similar design to Enterprise Storage with redundancy, battery 
> backup, etc, I'd
> be fine with it, but you don't. QEMU is a layer that I've also thought 
> was suppose
> to be small, lightweight and unobtrusive that is silently putting 
> everyones data
> at risk.
>
> The low-end iSCSI server from EqualLogic claims:
>     "it combines intelligence and automation with fault tolerance"
>     "Dual, redundant controllers with a total of 4 GB battery-backed 
> memory"
>
> AFAIK QEMU provides neither of these characteristics.

So if this is your only concern, we're in violent agreement.  You were 
previously arguing that we should use O_DIRECT in the host if we're not 
"lying" about write completions anymore.  That's what I'm opposing 
because the details of whether we use O_DIRECT or not have absolutely 
nothing to do with data integrity as long as we're using O_DSYNC.

Regards,

Anthony Liguori

> -mark
>
> > The fact that the virtualization layer has a cache is really not that 
> > unusual.
> Do other virtualization layers lie to the guest and indicate that the 
> data
> has successfully been ACK'd by the storage subsystem when the data is 
> actually
> still in the host cache?
>
>
> -mark
> > Regards,
> >
> > Anthony Liguori