VM Disk Expansion Guide (Proxmox + Linux)

Tags: #proxmox #storage #filesystem #resize

Quick Reference

Expanding a VM disk is a 3-step process: 1. Proxmox: Increase virtual disk size 2. Guest OS: Expand the partition 3. Guest OS: Resize the filesystem

Step 1: Increase Disk Size in Proxmox

Via Web UI

1. Select VM → Hardware
2. Click on Hard Disk (scsi0)
3. Click "Resize disk" button
4. Enter size to add (e.g., +50)
5. Click "Resize disk"

Via CLI (Proxmox Host)

# Increase VM 100's disk by 50GB
qm resize 100 scsi0 +50G

# Verify
qm config 100 | grep scsi0

Result: Virtual disk is larger, but guest OS doesn’t see it yet.

Step 2: Expand Partition (Inside VM)

Check Current Situation

# View disk and partition layout
lsblk

# Example output:
# sda      8:0    0   150G  0 disk       ← Disk is 150G
# └─sda2   8:2    0   100G  0 part /     ← Partition still 100G

Expand the Partition

Method A: Using growpart (Recommended)

# Install if needed
sudo apt install cloud-guest-utils

# Expand partition 2 on /dev/sda
sudo growpart /dev/sda 2

# Verify
lsblk
# sda2 should now show 150G

Method B: Using parted

# Interactive mode
sudo parted /dev/sda
(parted) print free          # Show layout
(parted) resizepart 2 100%   # Resize partition 2 to 100%
(parted) quit

# Non-interactive
sudo parted /dev/sda resizepart 2 100%

Method C: Using fdisk (Advanced)

# Only if above methods fail
sudo fdisk /dev/sda
# Type: d, 2 (delete partition 2)
# Type: n, p, 2, <enter>, <enter> (recreate larger)
# Type: w (write changes)
# Reboot if partition in use

Step 3: Resize Filesystem (Inside VM)

For ext4 Filesystem (Ubuntu/Debian)

# Resize filesystem to fill partition
sudo resize2fs /dev/sda2

# Verify
df -h /

For XFS Filesystem (CentOS/RHEL)

# Resize XFS filesystem
sudo xfs_growfs /

# Verify
df -h /

For Btrfs Filesystem

# Resize Btrfs filesystem
sudo btrfs filesystem resize max /

# Verify
df -h /

Complete Example Walkthrough

Scenario: Expand Ubuntu VM from 100GB to 150GB

Step 1: Proxmox Host

# Add 50GB to VM 100
qm resize 100 scsi0 +50G

Step 2: Inside Ubuntu VM

# Check current state
lsblk
df -h /

# Output shows:
# Disk: 150G, Partition: 100G, Filesystem: 99G

# Expand partition
sudo growpart /dev/sda 2

# Check partition expanded
lsblk
# Output now shows: Partition: 150G

# Expand filesystem
sudo resize2fs /dev/sda2

# Verify final result
df -h /
# Output now shows: Filesystem: 148G (150GB - overhead)

Success! Your VM now has 50GB more space.

Troubleshooting

Issue: Partition Won’t Expand

Symptom:

sudo growpart /dev/sda 2
# Error: no space to grow

Solution:

# Check if Proxmox resize worked
lsblk
# Look at sda size - should be larger

# If sda size is still old:
# - Shutdown VM
# - Resize again in Proxmox
# - Start VM

Issue: “GPT PMBR size mismatch” Warning

Symptom:

sudo fdisk -l /dev/sda
# GPT PMBR size mismatch (209715199 != 314572799)

Solution: This is normal and will be fixed automatically when you resize the partition.

# Fix GPT table
sudo parted /dev/sda
(parted) print
# Answer 'Fix' when prompted
(parted) quit

Issue: “Filesystem is mounted, can’t resize”

For ext4:

# ext4 supports online resizing
sudo resize2fs /dev/sda2
# This works while mounted

For XFS:

# XFS must be mounted to resize
sudo xfs_growfs /
# Uses mount point, not device

For others:

# May need to unmount or use live CD
# Boot from live USB
# Run resize2fs /dev/sda2
# Reboot

Issue: Partition Doesn’t Use Full Disk

Check unallocated space:

sudo parted /dev/sda print free

# Shows something like:
# Number  Start   End     Size    File system  Name
#  1      1049kB  2097kB  1049kB               
#  2      2097kB  107GB   107GB   ext4
#         107GB   161GB   53.7GB  Free Space   ← Unallocated

Solution: Expand the partition (Step 2 above)

Different Disk Naming Schemes

VM Disk Type	Linux Device	Usage
IDE	/dev/hda	Old, slow
SCSI	/dev/sda	Common
VirtIO Block	/dev/vda	Fastest (recommended)
VirtIO SCSI	/dev/sda	Fast + TRIM support

Adjust commands accordingly: - VirtIO: Use /dev/vda instead of /dev/sda - Multiple disks: /dev/sdb, /dev/sdc, etc.

Pre-Flight Checklist

Before expanding disk:

Backup VM - Take snapshot or vzdump backup
Check free space in Proxmox storage
Verify thin pool not nearly full: lvs pve/data

Note current sizes for verification:

# In Proxmox
qm config <vmid> | grep scsi0

# In VM
df -h /
lsblk

Post-Expansion Verification

# In VM - verify everything
lsblk                    # Disk and partition sizes
df -h                    # Filesystem size and usage
sudo parted /dev/sda print  # Partition table

# Should all show new larger sizes

Common Disk Sizes

Original	Add	Final	Use Case
32G	+18G	50G	Minimal server
100G	+50G	150G	Development VM
100G	+100G	200G	Production workload
200G	+300G	500G	Database/storage

Tip: Add space in increments. Easier to add more later than remove.

LVM Scenario (Different Process)

If your VM uses LVM:

# Check if LVM
lsblk
# Look for "lvm" in TYPE column

# Example with LVM
lsblk
# sda
# └─sda2              (physical partition)
#   └─ubuntu--vg-root (LVM volume)

# Resize process:
# 1. Proxmox: Increase disk size
qm resize 100 scsi0 +50G

# 2. VM: Expand partition
sudo growpart /dev/sda 2

# 3. VM: Resize physical volume
sudo pvresize /dev/sda2

# 4. VM: Expand logical volume
sudo lvextend -l +100%FREE /dev/ubuntu-vg/root

# 5. VM: Resize filesystem
sudo resize2fs /dev/ubuntu-vg/root

# Verify
df -h /

Quick Command Reference

# View disk layout
lsblk
lsblk -f                        # With filesystem info
sudo parted /dev/sda print      # Detailed partition info
sudo fdisk -l /dev/sda          # Alternative view

# Check filesystem
df -h                           # Mounted filesystems
df -h /                         # Root filesystem specifically
sudo tune2fs -l /dev/sda2       # ext4 details

# Expand operations
sudo growpart /dev/sda 2        # Expand partition
sudo resize2fs /dev/sda2        # Expand ext4
sudo xfs_growfs /               # Expand XFS
sudo btrfs filesystem resize max /  # Expand Btrfs

# Verify
lsblk && df -h /                # Quick check both

Real-World Example

Initial State:

# Proxmox: VM disk = 100GB
# VM: Partition = 100GB  
# VM: Filesystem = 99GB (22GB used)

Commands Used:

# 1. Proxmox (via web UI):
#    Hardware → Disk → Resize → +50

# 2. Ubuntu VM:
sudo apt install cloud-guest-utils
sudo growpart /dev/sda 2
sudo resize2fs /dev/sda2

Final State:

# Proxmox: VM disk = 150GB
# VM: Partition = 150GB
# VM: Filesystem = 148GB (22GB used, 120GB free)

Result: +49GB usable space gained! ✅

Last Updated: 2025-10-30
Tested On: Proxmox 9.0.3, Ubuntu 25.10
Status: Working procedure ✅

Vm Disk Expansion Guide Proxmox Linux