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

From: Carl-Daniel Hailfinger
Subject: Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error.
Date: Tue, 20 Jan 2009 11:44:57 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20080922 SUSE/1.1.12-0.1 SeaMonkey/1.1.12

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.
There's also an alternative for automatically resetting the mode after a
successful access: http://support.microsoft.com/kb/817472



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