Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error.

[Gleb Natapov <gleb@redhat.com>]

On Tue, Jan 20, 2009 at 11:44:57AM +0100, Carl-Daniel Hailfinger wrote:
> On 20.01.2009 10:35, Gleb Natapov wrote:
> > On Tue, Jan 20, 2009 at 10:25:04AM +0100, Gerd Hoffmann wrote:
> >   
> >> Ian Jackson wrote:
> >>     
> >>> Anthony Liguori writes ("Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error."):
> >>>       
> >>>> Ian Jackson wrote:
> >>>>         
> >>>>> Once again, this feature should be optional.
> >>>>>           
> >>>> Why?
> >>>>         
> >>> Well, three reasons, one general and theoretical, and two practical
> >>> and rather Xen-specific.
> >>>
> >>> The theoretical reason is that a guest is in a better postion to deal
> >>> with the situation because it knows its access patterns.  Often the
> >>> response to a failing write in a mission-critical system will be some
> >>> kind a fallback behaviour, which is likely to work.  Stopping the VM
> >>> unconditionally is not something that the guest can cope with.
> >>>       
> >> The fundamental issue is that you can't signal ENOSPC to the guest via
> >> IDE protocol because that is an error condition which simply can't
> >> happen on real hardware.  You can only signal EIO, which is something
> >> very different, and the OS likely goes into "Oops, disk broken" mode.
> >> Which probably isn't what you want here ...
> >>
> >>     
> > Windows using IDE retries DMA 3 times and then moves to PIO mode. And it
> > stays in PIO mode even after reboot. The only way to return to DMA mode
> > again is to reinstall the driver.
> >   
> 
> You can fix this in the registry without any reinstallation by setting
> the bitmask of allowed IDE modes back to 0xffffffff. It's a single value
> that needs to be changed. http://winhlp.com/node/10 has more info and a
> script to reset the bitmask.
Doesn't really matter how you re-enable DMA mode again, the fact is it
needs admin attention. Reinstalling the driver also not so hard. One
mouse click.

> There's also an alternative for automatically resetting the mode after a
> successful access: http://support.microsoft.com/kb/817472
> 
After 3 _consecutive_ errors IDE will still move to PIO mode and stay
there till admin intervention. And with ENOSPC you can be sure you'll
get 3 consecutive errors.

--
			Gleb.