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There has been some confusion on the topic of driver modes and 
share-server, especially as they related to storage controllers with 
multiple physical nodes, so I will try to clear up the confusion as much 
as I can.

Manila has had the concept of "share-servers" since late icehouse. This 
feature was added to solve 3 problems:
1) Multiple drivers were creating storage VMs / service VMs as a side 
effect of share creation and Manila didn't offer any way to manage or 
even know about these VMs that were created.
2) Drivers needed a way to keep track of (persist) what VMs they had created
3) We wanted to standardize across drivers what these VMs looked like to 
Manila so that the scheduler and share-manager could know about them

It's important to recognize that from Manila's perspective, all a 
share-server is is a container for shares that's tied to a share network 
and it also has some network allocations. It's also important to know 
that each share-server can have zero, one, or multiple IP addresses and 
can exist on an arbitrary large number of physical nodes, and the actual 
form that a share-server takes is completely undefined.

During Juno, drivers that didn't explicity support the concept of 
share-servers basically got a dummy share server created which acted as 
a giant container for all the shares that backend created. This worked 
okay, but it was informal and not documented, and it made some of the 
things we want to do in kilo impossible.

To solve the above problem I proposed driver modes. Initially I proposed 
3 modes:
1) single_svm
2) flat_multi_svm
3) managed_multi_svm

Mode (1) was supposed to correspond to driver that didn't deal with 
share servers, and modes (2) and (3) were for drivers that did deal with 
share servers, where the difference between those 2 modes came down to 
networking details. We realized that (2) can be implemented as a special 
case of (3) so we collapsed the modes down to 2 and that's what's merged 
upstream now.

The specific names we settled on (single_svm and multi_svm) were perhaps 
poorly chosen, because "svm" is not a term we've used officially 
(unofficially we do talk about storage VMs and service VMs and the svm 
term captured both concepts nicely) and as some have pointed out, even 
multi and single aren't completely accurate terms because what we mean 
when we say single_svm is that the driver doesn't create/destroy share 
servers, it uses something created externally.

So one thing I want everyone to understand is that you can have a 
"single_svm" driver which is implemented by a large cluster of storage 
controllers, and you have have a "multi_svm" driver which is implemented 
a single box with some form of network and service virtualization. The 
two concepts are orthagonal.

The other thing we need to decide (hopefully at our upcoming Jan 15 
meeting) is whether to change the mode names and if so what to change 
them to. I've created the following etherpad with all of the suggestions 
I've heard so far and the my feedback on each:
https://etherpad.openstack.org/p/manila-driver-modes-discussion

-Ben Swartzlander