VMware – Changed Block Tracking

This is just a brief post for my own reference. A friend of mine had some backup problems on a VM recently. Seems they were using VMware’s Changed Block Tracking (CBT) and had since added a physical-mode RDM to the VM. This caused problems and things stopped working. The quick solution was to clone the VM and delete the stale data. I’m sure there’s a more thorough solution. But I don’t like to scratch the surface too much. CBT has been around a while, and is a pretty nifty feature. The VMguy has a good post on CBT and how it works. Duncan has a bit more info. And VMware themselves have some useful KB articles here and here.

vCenter 2.5 and RDMs and multiple guests

While most of you were doing whatever it is you do to relax over the Easter long weekend, I was lucky enough to be cutting over a chunk of our environment with the help of SAN Copy. For the most part, things went well. The only major problem was the Solaris LDOM environment, but our very patient consultant sorted that out for us.

One issue I did have, however, was when I was cutting over RDM LUNs on a number of virtualised clusters. The problem was, basically, that after remapping the RDM on the first guest, I was unable to see the RDM files on the second guest. While some people in our environment believe it’s acceptable to run single-node clusters, I don’t.

It turns out that, and I can’t remember when exactly, the behaviour of vCenter changed to mask RDMs that are already presented to a guest. For those of you playing at home, we’re running the latest vCenter 2.5 (build 227637). So, I needed to add the following setting to the Advanced Settings in vCenter’s configuration. The setting is vpxd.filter.rdmFilter and it should be set to false. Also worthy of note is that this doesn’t seem to survive restarts of the vCenter service. But that’s probably because I’ve done something boneheaded.

Here’s what you need to do.

Then click on Add Row to add the desired settings and you’ll be able to add the RDMs to multiple guests.