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Re: [libvirt] [PATCH v3 4/7] qemu: Translate the iscsi pool/volume disk source



Il 23/07/2013 16:14, Daniel P. Berrange ha scritto:
> On Tue, Jul 23, 2013 at 03:52:56PM +0200, Paolo Bonzini wrote:
>> Il 23/07/2013 15:36, Daniel P. Berrange ha scritto:
>>> On Tue, Jul 23, 2013 at 03:29:40PM +0200, Paolo Bonzini wrote:
>>>> Il 23/07/2013 15:26, Daniel P. Berrange ha scritto:
>>>>>>>
>>>>>>> Ok, this answers my question. :)
>>>>>>>
>>>>>>> I think the default mode should be direct, because otherwise things such
>>>>>>> as persistent reservations do not work.
>>>>> No, the default has to be host mode, because that is the only mode that
>>>>> is guaranteed to be usable with any QEMU. The direct mode requires a
>>>>> QEMU that is new enough, and a distro to have enabled it. We can't
>>>>> rely on that as a default choice.
>>>>
>>>> Volume sources are also new enough that you can assume a good QEMU.
>>>
>>> No we can't assume that. New libvirt is frequently used with old QEMU
>>> and we want to have good default behaviour there.
>>
>> New libvirt, yes.  But can't new libvirt features also assume a new QEMU
>> if the old one causes more trouble than anything?
> 
> I've no problem with new features requiring new QEMU but that's not
> the issue here. This is a question about what the default configuration
> should be. For default configuration we aim for maximum compatibility
> not the latest possible features.

Shouldn't default configuration be simply "what makes the most sense"?

>> If you use host mode and the guest issues reservation commands, you may
>> get data corruption.  That is the brokenness.
> 
> So be it, that's been the case for every version of libvirt and QEMU
> in existance up until iscsi protocol support was added, so it is not
> a new issue. That doesn't justify choosing a default that is guaranteed
> to not work at all.

I started adding iSCSI support to ensure that iSCSI LUNs would work.
This makes them _not_ work by default.  It makes me regret asking to add
support for volumes as a disk source.

Paolo


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