Re: [Xen-devel] Driver domains communication protocol proposal

[George Dunlap <dunlapg@xxxxxxxxx>]

On Tue, Apr 24, 2012 at 7:00 PM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> George Dunlap writes ("Re: [Xen-devel] Driver domains communication protocol 
> proposal"):
>> On Wed, Apr 4, 2012 at 4:46 PM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> 
>> wrote:
>> >  vdi
>> >     This host's intent to access a specific target.
>> >     Non-persistent, created on request by toolstack, enumerable.
>> >     Possible states: inactive/active.
>> >     Abstract operations: prepare, activate, deactivate, unprepare.
>>
>> VDI as used by XenServer seems to mean "virtual disk instance", and as
>> such is actually persistent.  I don't quite understand what it's
>> supposed to mean here, and how it differs from VBD (which in XenServer
>> terminology means "virtual block device").
>
> One "vdi" in this sense can support multiple "vbd"s.  A "vbd"
> represents an attachment to a domain (or some other kind of provision
> for use) whereas a "vdi" is a preparatory thing.

Ah, so what you're calling "vdi" in this case is a thing into which
vbd's can plug -- what we might call the backend "node" for a
particular disk image?

So we have:

[A] <--> [B] <--> { [C], [D], [E] }

Where:
* A is the actual disk image on stable storage somewhere
* B is the instance of the code that can access A and provide access
to VMs which connect to it (not persistent)
* C D and E are instances of code running inside the guest which
connect to B and provide a block device to the guest OS which looks
like A (again not persistent)

Is that correct?

I think calling A a "virtual disk image" makes the most sense; reusing
that name for B is a bad idea given that it's already used for A in
XenServer terminology.  (Jonathan, correct me if I'm wrong here.)

I think that calling C D and E "vbd"s also makes sense.

So we just need to have a good name for the running instance of a
blockback process / thread / whatever that accesses a particular VDI.
Virtual disk provider (VDP)? Block back instance (BBI)?  Virtual block
backend (VBB)?

 -George

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