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[Xen-devel] [Question] Using backend snapshots with a storage repository


I'm a software engineer at a data-storage company in Boulder called SolidFire.

I work on the Apache CloudStack project on behalf of my company to help drive CloudStack's storage architecture forward.

I have a question with regards to storage repositories and backend (SAN, in this case) snapshots.

If I have a storage repository on a LUN, I can take a snapshot of the LUN using my company's snapshot technology.

A problem comes into play, however, when I try to attach this new LUN to the same Xen cluster as the LUN it is a snapshot of.

Xen does not like the fact that both SRs have the same UUID (and, in fact, VDIs in each SR have duplicate UUIDs).

It appears UUIDs are immutable, so I'm not sure how to cleanly get around this problem.

Since I write code that supports multiple hypervisors in CloudStack, I also came across this issue in ESXi.

Here is ESXi's solution:

At the time being, I cannot use a backend-snapshot mechanism for Xen. What I do is create a new LUN, a new SR on this LUN, take a VM snapshot of the VDI I want that's on the source SR, then copy the VM snapshot of the VDI to the new SR (then delete the VM snapshot of the VDI on the source SR).

Even though our SAN de-duplicates the storage blocks (so not a lot of new data is written to the SAN), this workaround takes up front-end CPU cycles and network bandwidth (and a noticeable amount time compared to backend snapshots, which can be almost instantaneous).

What do you recommend?


Mike Tutkowski
Senior CloudStack Developer, SolidFire Inc.
o: 303.746.7302
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