Re: [PATCH RFC v1] libxl: Introduce a template for devices with a controller

[George Dunlap <george.dunlap@eu.citrix.com>]

On 05/27/2015 11:09 AM, Juergen Gross wrote:
> George,
> 
> I'm on vacation this and the next week with only limited email access.
> So please don't expect fast reaction on any further questions during
> this time. :-)

Then quit reading your work e-mail and get back to the important stuff! :-)

>> OK, so I looked it up[1] and the full address seems to be:
>> * adapter number / host
>> * channel number / bus
>> * id number / target
>> * LUN
>>
>> In which case, "controller" would correspond to "adapter / host", right?
>>
>> In the vscsi world, what levels of what can you make?  I know you
>> mentioned before that some devices have multiple LUNs, and those need
>> to be grouped together, with the same LUNs as they do on real
>> hardware, to work properly -- is that right?
> 
> Not all of the devices have this requirement, but some.
> 
>> The USB case actually has something somewhat similar:
>> * USB controller
>> * USB bus
>> * USB device
>> * USB function
>>
>> So far, there's not really a controller/bus distinction: each
>> controller has exactly one bus.  When we assign a USB device to a bus,
>> we automatically go through and assign each function fo that device
>> individually.
>>
>> Would it make sense to treat vscsi the same way -- i.e., to make a
>> "bus", and then attach "targets"s to it, and have the LUNs for any
>> given target automatically assigned when the target is assigned?
> 
> As long as it is still possible to assign individual LUNs as well.
> If dom0 is controlling e.g. a RAID system you might want to assign
> one LUN of a target to domU A and one LUN of the same target to domU B.

OK, so it sounds like in the vscsi case, it would be useful to assign
either an entire target, or an individual LUN.

In the case of assigning a target, you'll want to assign all the LUNs as
well, such that the virtual LUNs mirror the real LUNs.

In the case of assigning a LUN, I assume you'll still need a virtual
target.  Will you be wanting an interface for creating virtual targets,
so that you can assign several real LUNs to the same target?  Or will
you just want one virtual target per LUN if you're not assigning an
entire target?

 -George