How to Remove a Data Disk from an Azure VM (How not to blow your leg off)

I was working with a client recently, were we had to reconfigure storage within a VM (which is always a messy proposition). In doing so, we were adding and removing disks from the VM. this all happened mostly during a downtime window, so it wasn’t a big deal to down a VM, which is how you can remove a disk from a VM via the portal. However, upon further research, I learned that through the portal you can remove a disk from a running VM.

For the purposes of this demo, I’ve built a a SQL Server VM with two data disks and a single disk for transaction log files. The SQL VMs use Storage Spaces in Windows, which is a recommended best practice–but even if you are not using Storage Spaces, most of this will apply.

How To Identify Your Disk

This is the really important part of this post–how to identify what your disk is in the portal and with your VM. When you define a data disk in the portal, either you or the system will define a LUN number for the individual disk. You can see it on the portal in the below screenshot.

This number is mostly meaningless, except that within Windows, it lets you identify the disk. If you open up Server Manager and navigate to Storage Pools > Physical Disk, you can see where this LUN number shows up.

That number maps back to the values you see in the Azure portal, and unless you size each of your disks differently (which you shouldn’t do for performance reasons). If you aren’t using Storage Spaces, you can also see the LUN number in Disk Management in Windows as shown below.

You can also get this information using PowerShell using the Windows cmdlet Get-PhyiscalDisk.

It is very important to ensure that you have identified the correct disk before you remove it.

Removing the Disk

Azure will let you delete a disk from a running VM. Even if that disk is mounted in the VM and has data on it. Yes, I just did this on my test VM.

If you click one of those highlighted Xs and then click Save, your disk will be removed from your VM. There’s also a series of PowerShell commands you can use to do this. It is also important to note, that at this point your disk is still an Azure resource. Even though you have removed it from the VM, the disk still exists and has all the data it had the moment you detached it from the VM.

If you chose the correct disk to remove from your VM, and you have confirmed that your VM is healthy, you can navigate into the resource group for your VM where you will see your disks.

The important thing to note is that the state of the disk is “Unattached”, which means it’s not connected to a VM. So it can be deleted from Azure–I don’t recommend doing so until you have validated your VMs are running as expected.

You may ask how you can prevent disks from being removed from running VMs. I’ll write a post about this next week, but while you are waiting read up on resource locks in Azure.

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