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

From: Gleb Natapov
Subject: Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error.
Date: Tue, 20 Jan 2009 11:35:40 +0200

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.


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