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. 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?