[openstack-dev] [Manila] Driver modes, share-servers, and clustered backends
ben at swartzlander.org
Thu Jan 8 22:02:00 UTC 2015
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
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
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:
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