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



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.
> 
> Paolo
> 

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


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?

John


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