Enhanced RBD support for CloudStack 4.2

About 1 hour ago the new storage subsystem got merged into the master branch of CloudStack. That is wonderful news for all you out there who want to use features like snapshotting with RBD in CloudStack.

In pre-4.2 CloudStack a snapshot was the same as a backup. As soon as you created a snapshot it would also copy that snapshot to the secondary storage. This could not only lead to high network utilization when talking about 1TB RBD volumes, but it also caused problems with the underlying ‘qemu-img’ tool. To make a long story short: Snapshots with RBD just wouldn’t work in CloudStack 4.0 or 4.1 without resorting to dirty hacking. Which we didn’t.

The new storage subsystem separates the backup and snapshot process. Snapshots are handled by the primary storage and they can be copied to the ‘backup storage’ on request. This allows is to use the full snapshot potential of RBD.

I was waiting for the storage subsystem to be merged into the master branch before I could start working on this. About two weeks ago I already wrote a small function spec in CloudStack’s wiki to describe what has to be done.

A couple of choices still have to be made. Traditionally we could do everything through libvirt and ‘qemu-img’, but from what I can see now we’ll run into some trouble. We might have to go through the process of wrapping librbd into a Java library to get it all done, but I’m not completely positive about that. Some patches for libvirt(-java) could probably also do the job, but it would take a lot of time and work to get those upstream and into the repositories. The goal is to have this new RBD code work natively on a Ubuntu 13.04 system.

The expectation is that CloudStack 4.2 will be released mid-July this year, but if you are a daredevil you can always track the master branch and play around with that.

I’ll post updates on the cloudstack-dev list on a regular base about the progress, but you can also watch the master branch and search for commits with ‘RBD’ in the message.

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Wido den Hollander

I am the CTO at PCextreme B.V. and i will keep this blog just to post some interesting information about my daily work on Linux (and other) systems.