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[Xen-devel] Re: [PATCH] Fix restore handling checks

To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] Fix restore handling checks
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date: Wed, 23 Jun 2010 13:19:01 +0200
Cc: Michal Novotny <minovotn@xxxxxxxxxx>, "'xen-devel@xxxxxxxxxxxxxxxxxxx'" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On 06/22/2010 04:56 PM, Ian Jackson wrote:
Michal Novotny writes ("Re: [Xen-devel] [PATCH] Fix restore handling checks"):
Right, there's the exception for CD-ROMs according to the spec. I should
implement this as well but read-only IDE disk devices are not supported
according to the IDE specs.

I don't think this is a very good argument for not supporting them.

The Xen administrator should be able to present a guest with an IDE
disk and nevertheless prevent the guest from writing to it.

If that means that the guest administrator needs to be told
out-of-band that writes will not work, rather than having their kernel
automatically discover the read-only nature, then that's fine.  And if
the guest administrator doesn't do that and their kernel tries to
write and gets IDE errors then that is the correct consequence.

We tried to get that to work, and we could not really find a device error that worked reliably in the guests. We often got infinite loops on mount or little after that, on both Linux and Windows.

If you want to use read-only devices, you should really use SCSI or PV devices.

Paolo

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