Resizing the Linux Root Partition in a Gen2 Hyper-V VM

Resizing the Linux Root Partition in a Gen2 Hyper-V VM

Without a doubt, modern virtualization has changed the landscape of enterprise computing forever. Since virtual machines are abstracted away from the physical hardware, changes in compute, memory, and storage resources become mere clicks of a mouse. And, as hypervisors mature, many operations that were once thought of as out-of-band tasks, such as adding storage or even memory can now be done with little, or even zero downtime.

Hyper-V SCSI Disks and Linux

In many cases, hypervisors are backed by large storage area networks (SANs). This provides shared storage for hypervisor nodes that supports failover clustering and high availability. Additionally, it gives administrators the ability to scale the virtual environment, including the ability to easily add or expand storage on existing virtual servers. Microsoft’s Hyper-V 2012 introduced Generation 2 VMs, which extends this functionality. Among the many benefits of Gen2 VMs, was the ability to boot from a SCSI disk rather than IDE. This requires UEFI rather than a legacy BIOS, so it’s only supported among newer operating systems. Many admins I talk to think this is limited to Microsoft Server 2012 and newer, probably because of the sub-optimal phrasing in the Hyper-V VM creation UI that altogether fails to mention Linux operating systems.

The fact is, however, that many newer Linux OSes also support this ability, as shown in these tables from Microsoft.

More Disk, Please

Once you’ve built a modern Linux VM and you’re booting from synthetic SCSI disks rather than emulated IDE drives, you gain numerous advantages, not the least of which is the ability to resize the OS virtual hard disk (VHDX) on the fly. This is really handy functionality – after all, what sysadmin hasn’t had an OS drive run low on disk space at some point in their career? This is simply done from the virtual machine settings in Hyper-V Manager or Failover Cluster Manager by editing the VHDX.

Now, if you’re a Microsoft gal or guy, you already know that what comes next is pretty straightforward. Open the Disk Management MMC, rescan the disks, extend the file system, and viola, you now automagically have a bigger C:\ drive. But what about for Linux VMs? Though it might be a little less intuitive, we can still accomplish the same goal of expanding the primary OS disk with zero down time in Linux.

On-the-Fly Resizing

To demonstrate this, let’s start with a vanilla, Hyper-V Generation 2, CentOS 7.6 VM with a 10GB VHDX attached to a SCSI controller in our VM. Let’s also assume we’re using the default LVM partitioning scheme during the CentOS install. Looking at the block devices in Linux, we can see that we have a 10GB disk called sda which has three partitions – sda1, sda2 and sda3. We’re interested in sda3, since that contains our root partition, which is currently 7.8GB, as demonstrated here by the lsblk command.

Now let’s take a look at df. Here we can see an XFS filesystem on our 7.8GB partition, /dev/mapper/centos-root which is mounted on root.

Finally, let’s have a look at our LVM summary:

From this information we can see that there’s currently no room to expand our physical volume or logical volume, as the entirety of /dev/sda is consumed. In the past, with a Gen1 Hyper-V virtual machine, we would have had to shut the VM down and edit the disk, since it used an emulated IDE controller. Now that we have a Gen2 CentOS VM with a SCSI controller, however, we can simply edit the disk on the fly, expanding it to 20GB.

Once the correct virtual disk is located, select the “Expand” option.

Next, provide the size of the new disk. We’ll bump this one to 20GB.

Finally, click “Finish” to resize the disk. This process should be instant for dynamic virtual hard disks, but may take a few seconds to a several minutes for fixed virtual hard disks, depending on the size of the expansion and speed of your storage subsystem. You can then verify the new disk size by inspecting the disk.

OK, so we’ve expanded the VHDX in Hyper-V, but we haven’t done anything to make our VM’s operating system aware of the new space. As seen here with lsblk, the OS is indifferent to the expanded drive.

Taking a look at parted, we again see that our /dev/sda disk is still showing 10.7GB. We need to make the CentOS operating system aware of the new space. A reboot would certainly do this, but we want to perform this entire operation with no downtime.

Issue the following command to rescan the relevant disk – sda in our case. This tells the system to rescan the SCSI bus for changes, and will report the new space to the kernel without a restart.

Now, when we look at parted again, we’re prompted to move the GPT table to the back of the disk, since the secondary table is no longer in the proper location after the VHDX expansion. Type “Fix” to correct this, and then once again to edit the GPT to use all the available disk space. Once this is complete, we can see that /dev/sda is now recognized as 20GB, but our sda3 partition is still only 10GB.

Next, from the parted CLI, next use the resizepart command to grow the partition to the end of the disk.

Our sda3 partition is now using the maximum space available, 20.2GB. The lsblk command also now correctly reports our disk as 20GB.

But what about our LVM volumes? As suspected, our physical volumes, volume groups and logical volumes all remain unchanged.

We need to first tell our pv to expand into the available disk space on the partition. Do this with the pvresize command as follows:

Sure enough, our pv is now 18.8GB with 10.00GB free. Now we need to extend the logical volume and it’s associated filesystem into the free pv space. We can do this with a single command:

Looking at our logical volumes confirms that our root lv is now 17.80GB of the 18.80GB total, or exactly 10.0GB larger than we started with, as one would expect to see.

A final confirmation with the df command illustrates that our XFS root filesystem was also resized.


So there you have it. Despite some hearsay to the contrary, modern Linux OSes run just fine as Gen2 VMs on Hyper-V. Coupled with a SCSI disk controller for the OS VHDX, this yields the advantage of zero-downtime root partition resizing in Linux, though it’s admittedly a few more steps than a Windows server requires. And though Linux on Hyper-V might not seem like the most intuitive choice to some sysadmins, Hyper-V has matured significantly over the past several releases and is quite a powerful and stable platform for both Linux and Windows. And one last thing – when you run critically low on disk space on Linux, don’t forget to check those reserved blocks for a quick fix!

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