[openstack-dev] [cinder] consistency groups in ceph

Victor Denisov vdenisov at mirantis.com
Tue Nov 1 03:29:27 UTC 2016


I'm working on consistency groups feature in ceph.
My question is about what kind of behavior does cinder expect from
storage backends.
I'm particularly interested in what happens to consistency groups
snapshots when I remove an image from the group:

Let's imagine I have a consistency group called CG. I have images in
the consistency group:
Im1, Im2, Im3, Im4.
Let's imagine we have snapshots of this consistency group:


Snapshots of individual images in a consistency group snapshot I will call
CGSnap2Im1 - Snapshot of image 1 from consistency group snapshot 2.

Qustion 1:
If consistency group CG has 4 images: Im1, Im2, Im3, Im4.
Can CGSnap1 have more images than it already has: Im1, Im2, Im3, Im4, Im5.

Can CGSnap1 have less images than it already has: Im1, Im2, Im3.

Question 2:
If we remove image2 from the consistency group. Does it mean that
snapshots of this image should be removed from all the CGSnaps.

We are removing Im2.
CGSnaps look like this:

CGSnap1 - CGSnap1Im1, CGSnap1Im2, CGSnap1Im3
CGSnap2 - CGSnap2Im1, CGSnap2Im2, CGSnap2Im3, CGSnap3Im4
CGSnap3 - CGSnap3Im1, CGSnap3Im2, CGSnap3Im3, CGSnap3Im4

What happens to snapshots: CGSnap1Im2,CGSnap2Im2, CGSnap3Im2? Do we
remove them, do we keep them. Is it important what we do to them at


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