How to detach a data disk from a Linux virtual machine

Applies to: ✔️ Linux VMs ✔️ Flexible scale sets

When you no longer need a data disk that's attached to a virtual machine, you can easily detach it. This removes the disk from the virtual machine, but doesn't remove it from storage. In this article, we are working with an Ubuntu LTS 16.04 distribution. If you are using a different distribution, the instructions for unmounting the disk might be different.

If you want to use the existing data on the disk again, you can reattach it to the same virtual machine, or another one.

Connect to the VM to unmount the disk

Before you can detach the disk using either CLI or the portal, you need to unmount the disk and removed references to if from your fstab file.

Connect to the VM. In this example, the public IP address of the VM is 10.0.1.4 with the username azureuser:

ssh azureuser@10.0.1.4

First, find the data disk that you want to detach. The following example uses dmesg to filter on SCSI disks:

dmesg | grep SCSI

The output is similar to the following example:

[    0.294784] SCSI subsystem initialized
[    0.573458] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
[    7.110271] sd 2:0:0:0: [sda] Attached SCSI disk
[    8.079653] sd 3:0:1:0: [sdb] Attached SCSI disk
[ 1828.162306] sd 5:0:0:0: [sdc] Attached SCSI disk

Here, sdc is the disk that we want to detach. You also should grab the UUID of the disk.

sudo -i blkid

The output looks similar to the following example:

/dev/sda1: UUID="11111111-1b1b-1c1c-1d1d-1e1e1e1e1e1e" TYPE="ext4"
/dev/sdb1: UUID="22222222-2b2b-2c2c-2d2d-2e2e2e2e2e2e" TYPE="ext4"
/dev/sdc1: UUID="33333333-3b3b-3c3c-3d3d-3e3e3e3e3e3e" TYPE="ext4"

Edit the /etc/fstab file to remove references to the disk.

Open the /etc/fstab file in a text editor and remove the line containing the UUID of your disk. Using the example values in this article, the line would look like the following:

UUID=33333333-3b3b-3c3c-3d3d-3e3e3e3e3e3e   /datadrive   ext4   defaults,nofail   1   2

Save and close the file when you're done.

Next, use umount to unmount the disk. The following example unmounts the /dev/sdc1 partition from the /datadrive mount point:

sudo umount /dev/sdc1 /datadrive

Detach a data disk using Azure CLI

This example detaches the myDataDisk disk from VM named myVM in myResourceGroup.

az vm disk detach -g myResourceGroup --vm-name myVm -n myDataDisk

The disk stays in storage but is no longer attached to a virtual machine.

Lower latency

In select regions, the disk detach latency has been reduced, so you'll see an improvement of up to 15%. This is useful if you have planned/unplanned failovers between VMs, you're scaling your workload, or are running a high scale stateful workload such as Azure Kubernetes Service. However, this improvement is limited to the explicit disk detach command, az vm disk detach. You won't see the performance improvement if you call a command that may implicitly perform a detach, like az vm update. You don't need to take any action other than calling the explicit detach command to see this improvement.

Detach a data disk using the portal

1. In the left menu, select Virtual Machines.
2. In the virtual machine blade, select Disks.
3. In the Disks blade, to the far right of the data disk that you would like to detach, select the detach button, to detach the disk.
4. After the disk has been removed, select Save on the top of the blade.

The disk stays in storage but is no longer attached to a virtual machine. The disk is not deleted.

Next steps

If you want to reuse the data disk, you can just attach it to another VM.

If you want to delete the disk, so that you no longer incur storage costs, see Find and delete unattached Azure managed and unmanaged disks - Azure portal.