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:

-Ben Swartzlander

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