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



Il 23/07/2013 18:47, John Ferlan ha scritto:
> On 07/23/2013 12:18 PM, Paolo Bonzini wrote:
>> Il 23/07/2013 18:01, Daniel P. Berrange ha scritto:
>>> On Tue, Jul 23, 2013 at 05:35:57PM +0200, Paolo Bonzini wrote:
>>>> Il 23/07/2013 16:14, Daniel P. Berrange ha scritto:
>>>>>>> Perhaps the default could be specified in a configuration file (and the
>>>>>>> default should be the safe one).
>>>>> No, that is even worse because now the default is not predictable..
>>>>>
>>>>> We simply default to host mode and if applications want to use the
>>>>> other mode they can configure the XML as desired.
>>>>
>>>> Can we just forbid mode='default' for iSCSI and force the user to
>>>> specify host vs. direct?
>>>
>>> That would mean that apps cannot simply configure a guest volume
>>> without first checking to find out what type of pool it is, and
>>> then specifying this extra arg for iSCSI. IMHO the value of the
>>> <volume> XML is that you don't have to know anything about the
>>> pool to be able to configure it - we're completely decoupled.
>>
>> Thinking more about it, it would only be needed for <disk type='volume'
>> device='lun'>.  And for that case, some knowledge of the pool is
>> necessary anyway (for one thing, it won't work with filesystem or LVM
>> pools).  So if we could forbid mode='default' for that case only, it
>> would be enough as far as I'm concernde.
> 
> Using "default" in the mode field would result in the following XML
> error message (I just quickly changed a test to prove the point):
> 
> 121) QEMU XML-2-XML disk-source-pool-mode
> ... libvirt: Domain Config error : XML error: unknown source mode
> 'default' for volume type disk
> FAILED

Sorry, by mode='default' I really meant "no mode at all" (I was under
the false impression that you could also specify a mode='default' with
the same effect as no mode at all).

> The XML parsing code only looks for "mode='direct'" or "mode='host'". If
> "mode" isn't present in the XML, that's when that default comes into
> play. Since 'mode' is new there could be configurations where its not in
> an XML file, thus a 0 (zero e.g. "default") value is provided for the field.
> 
> Once the XML is parsed we still needed a default when it's going to be
> added, thus the "magic" to set the default to HOST is in qemu_conf.c in
> qemuTranslateDiskSourcePool():
> 
> 
>      if (pooldef->type == VIR_STORAGE_POOL_ISCSI) {
>          /* Default to use the LUN's path on host */
>          if (!def->srcpool->mode)
>              def->srcpool->mode = VIR_DOMAIN_DISK_SOURCE_POOL_MODE_HOST;
> 
> 
> I think this answers your primary concern - correct?

No, my concern is that mode='host' is a bad default for the device='lun'
case.

Paolo


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