When you’re setting up Oracle’s Automatic Storage Management feature (ASM), you have to ensure that the ‘raw’ devices that you have added to your server for ASM’s use are assigned the correct device names and usable permissions every time the server bounces.

In the dark days of Red Hat 3 and 4, we generally arranged for that to happen by creating raw device mappings -but that’s no longer supported on RH5 or RH6. Instead, you’re supposed to create new udev rules which do the job of declaring which devices exist and which permissions they should at every server boot.

And one of the first things you have to do to write a decent udev rule is to correctly identify the hard disks that exist: you can’t apply a permissions rule to something which you can’t uniquely identify in the first place, after all.

So when this happens, you have a bit of a problem:

That’s four SCSI hard disks, previously added to my VMware Workstation virtual machine and partitioned, resolutely failing to respond to the scsi_id command, which is what you’re supposed to use to get a unique ‘id string’ returned for a device. This happens when I use virtual machines built on VMware’s ESXi 4.1 server, too.

But, funnily enough, it doesn’t happen if you use VirtualBox as your virtualization platform:

Now, I’m not going to say this proves VirtualBox is better than VMware (because it’s not), but I am going to point out that, by default, all VMware virtual machines exhibit this behaviour, which will stop you dead in your tracks if you’re trying to build a virtual ASM or RAC machine. Without that ID string, you can’t identify your ASM disks uniquely -and that means you can’t get those disks correctly discovered by the operating system …and it’s downhill all the way after that.

Lucky, then, that this is all fixable with a modest bit of re-configuration!

If you were running ESXi 4.1, that bit of re-configuration consists of

• shut down your virtual machine
• right-click the VM’s entry in the left-hand panel and select Edit Settings
• click the Options tab
• Select the Advanced -> General item on the left and click the Configuration Parameters… button you then see displayed on the right
• Click the Add Row button
• Add disk.EnableUUID as the name of the new row, and the word TRUE as its value (don’t use quotation marks around either of these entries).
• Click OK to make the new parameter addition ‘stick’.

You can then reboot your virtual machine.

Sadly, VMware Workstation has no interface like this that allows you to add this new configuration key to your VMs. Instead, you are reduced to having to do it yourself, by hand, using a text editor. It’s easy enough, however.

First, find the directory where the files representing your virtual machine are stored. One of them will be called the name of your VM, with an extension of .vmx. In my case, for example, the file is called sl6.vmx, because when I created my VM, I called it “SL6″. The file will be 3 or 4KB in size. Open it in the text editor of your choice and at the very end of the file, add this line:

disk.EnableUUID = "TRUE"

The quotation marks around the word “TRUE” here are important and must be typed. Now save the edited file and reboot your virtual machine. You should find that scsi_id is now capable of returning perfectly usable values:

As you can see from this screenshot, my VMware virtual machine is now displaying SCSI IDs for my hard disks just fine. The values being returned are quite different from those shown earlier in my VirtualBox VM, of course, because VMware and VirtualBox handle virtual disk identification quite differently. Whatever the specific values might be, though, the important point is to be able to see a value of any sort!