Expand Disk Space on a VMware Linux VM Without Rebooting

Say goodbye to downtime when you need more storage

Running out of disk space on a production VM is stressful enough without the added headache of scheduling a reboot. The good news? If you're running Linux on VMware, you can expand your disk and have the guest OS recognise the new space immediately — no reboot required.

In this guide, I'll walk you through the entire process, from expanding the VMDK in vSphere to resizing your filesystem inside the VM.

Understanding the Problem

When you increase the size of a virtual disk in VMware, the hypervisor knows about the change immediately. However, the Linux guest operating system doesn't automatically detect this change. It still sees the old disk size because the SCSI device information is cached.

Traditionally, a reboot would force Linux to re-read the disk geometry. But in production environments, reboots mean downtime — and downtime means unhappy users.

The Solution: SCSI Rescan

Linux provides a way to force the kernel to re-read disk information through the /sys filesystem. There are two scenarios to consider:

Scenario 1: Resizing an Existing Disk

If you've expanded an existing VMDK, you need to tell the specific block device to refresh its boundary information:

echo 1 > /sys/block/sda/device/rescan

Replace sda with your actual device name (e.g., sdb, sdc). This command tells the SCSI block device to refresh its information about where its ending boundary is, giving the kernel updated size information.

Scenario 2: Adding a New Disk

If you've hot-added a completely new disk to the VM, you need to scan the SCSI host adapter for new devices:

echo "- - -" > /sys/class/scsi_host/host0/scan

The three dashes represent wildcards for channel, target, and LUN respectively. To scan all adapters at once:

echo "- - -" | tee /sys/class/scsi_host/*/scan

Step-by-Step: Expanding an Existing Disk

Here's the complete workflow for expanding an existing disk on a running Linux VM:

Step 1: Expand the VMDK in vSphere

1. Open the vSphere Client
2. Right-click your VM and select Edit Settings
3. Find the hard disk you want to expand
4. Increase the size to your desired capacity
5. Click OK

Step 2: Rescan the Disk in Linux

First, identify which device you're working with:

lsblk

Then trigger a rescan on that device:

echo 1 > /sys/block/sda/device/rescan

Verify the new size is detected:

fdisk -l /dev/sda

Step 3: Expand the Partition

If you're using growpart (recommended), this is straightforward:

# Install growpart if needed (part of cloud-utils-growpart package)
yum install cloud-utils-growpart   # RHEL/CentOS
apt install cloud-guest-utils      # Debian/Ubuntu

# Grow the partition (partition 2 in this example)
growpart /dev/sda 2

Step 4: Resize the Filesystem or LVM

For LVM setups, resize the physical volume first:

pvresize /dev/sda2

Then extend your logical volume:

lvextend -l +100%FREE /dev/mapper/vg_name-lv_name

For ext4 filesystems:

resize2fs /dev/mapper/vg_name-lv_name
# Or for a direct partition:
resize2fs /dev/sda2

For XFS filesystems (common on RHEL/CentOS 7+):

xfs_growfs /mountpoint

Step 5: Verify the Changes

Confirm everything worked:

df -h

You should see your filesystem now reporting the expanded size.

Quick Reference Commands

Important Caveats

• Shrinking is different: This process is for expanding disks. Shrinking requires different steps and carries more risk
• Backups first: Always have a backup before modifying disk structures, even if the procedure is well-tested
• VMware Tools: Ensure VMware Tools is installed and running for best results

Conclusion

Expanding disk space on a VMware Linux VM without rebooting is not only possible — it's straightforward once you understand the process. The key insight is that Linux caches disk geometry, and the /sys filesystem gives us a way to force a refresh.

By combining VMware's hot-expand capability with Linux's SCSI rescan feature, you can handle storage emergencies without any downtime. Your users will thank you, and you'll sleep better knowing you have this technique in your toolkit.

Have questions or tips of your own? Drop them in the comments below!