Open Stack

For those of you looking for more details, here are the specs for each
feature:

1. Replication - https://review.openstack.org/#/c/98308/
2. Consistency Groups - https://review.openstack.org/#/c/96665

Replication:
- Looks like this is specifically for replication between two cinder-volume
servers with the same storage driver. Which is ok for me.
- I am concerned with the driver API around promoting replicas (e.g. only
calling a function on the secondary) since you are probably in a network
partition when this is called.

Consistency Groups:
- It looks like this proposal layers two concepts in consistency group
creation:
    * Marking what volume-types may be within the same consistency group.
(Operator issue I argue.)
    * Declaring a new consistency group with volumes in it. (User issue).
- Can you create a consistency group with volumes from separate regions?
- Can you create a consistency group with volumes from separate store
backends? What about different drivers?
- The current proposal will allow multiple volumes to be snapshotted at
around the same time and make use of QEMU quiesce functions to ensure that
the filesystems are in a good state when the snapshot has been made. Does a
separate consistency group snapshot function provide any stronger
consistency guarantees than simply calling snapshot on each volume in a for
loop? My impression based on this implementation is no.
- Then do we need a separate API, as indicated in the "alternatives"
section? This essentially becomes a kind of constraint on deleting volumes.
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