When deploying virtual machines on Proxmox VE, one of the most important decisions administrators make is how to configure and manage virtual machine disks. The choice of disk format and backend impacts not only performance, but also scalability, backup strategy, and snapshot capabilities.

Proxmox VE offers a flexible storage model that supports multiple disk types and formats, allowing administrators to tailor storage configurations to fit testing environments, production workloads, or high-availability clusters.

In this article, we’ll explore the VM disk formats supported in Proxmox VE, their advantages and trade-offs, and best practices for choosing the right one.

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Proxmox VE Storage Abstraction

Before diving into disk formats, it’s useful to understand how Proxmox handles storage. VM disks are created on top of storage backends defined in the storage.cfg. These can be:

• Local storage – directory, LVM, LVM-thin, ZFS.
• Shared storage – NFS, iSCSI, Ceph RBD, GlusterFS, etc.

Each storage type supports different disk image formats and features.

 

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VM Disk Formats Supported in Proxmox VE

1. RAW Disks

• Extension: .raw
• Description: A simple, unstructured disk format that maps directly to blocks on the underlying storage.
• Advantages:
  • Very fast (minimal overhead).
  • Works well with block storage backends like LVM, ZFS, and Ceph.
  • Fully compatible with snapshotting on ZFS/Ceph.
• Drawbacks:
  • No built-in compression.
  • Fixed size – does not support thin provisioning on directory storage.

Use case: Best choice for production when using block storage (LVM-thin, ZFS, Ceph) where snapshots and thin provisioning are handled by the backend.

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2. QCOW2 (QEMU Copy-On-Write v2)

• Extension: .qcow2
• Description: A flexible disk image format supported by QEMU/KVM, offering advanced features.
• Advantages:
  • Supports thin provisioning.
  • Built-in compression.
  • Supports snapshots.
  • Can grow dynamically as data is written.
• Drawbacks:
  • Slightly slower than RAW due to metadata overhead.
  • Fragmentation can degrade performance over time.
  • Not ideal for high-performance production workloads.

Use case: Useful for labs, test environments, and scenarios where flexibility and space efficiency are more important than raw performance.

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3. VMDK (VMware Disk)

• Extension: .vmdk
• Description: A disk format widely used in VMware environments.
• Advantages:
  • Enables migration of workloads between VMware and Proxmox.
  • Useful for importing/exporting VMs.
• Drawbacks:
  • Not as feature-rich on Proxmox compared to VMware.
  • Limited snapshot support.
  • Performance overhead compared to RAW.

Use case: Best for migration scenarios when moving workloads from VMware to Proxmox or maintaining compatibility in hybrid environments.

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4. VHD / VHDX (Virtual Hard Disk)

• Extension: .vhd, .vhdx
• Description: Formats commonly used by Microsoft Hyper-V. Proxmox supports importing these disks for migration.
• Advantages:
  • Migration-friendly if moving from Hyper-V.
• Drawbacks:
  • Not natively optimized for Proxmox.
  • Typically converted to RAW or QCOW2 for production use.

Use case: Primarily for importing VMs from Hyper-V into Proxmox VE.

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Special Disk Backends in Proxmox

Apart from image formats, Proxmox also integrates with storage technologies that handle disks differently:

1. LVM / LVM-Thin:
   • VM disks are created as logical volumes.
   • Thin provisioning available with LVM-Thin.
   • Excellent performance but no compression.
2. ZFS Volumes (ZVOLs):
   • VM disks stored as block devices on ZFS datasets.
   • Benefit from ZFS snapshots, replication, and compression.
   • Highly recommended for environments requiring data integrity.
3. Ceph RBD (RADOS Block Device):
   • Distributed block storage for clusters.
   • Ideal for HA setups with live migration.
   • Supports snapshots and replication across nodes.

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Choosing the Right VM Disk Format

Here’s a quick guide to selecting the best format for your use case:

Format / Backend	Performance	Snapshots	Thin Provisioning	Best Use Case
RAW	★★★★★	Backend-based	Backend-based	Production workloads, ZFS, LVM, Ceph
QCOW2	★★★★☆	Yes	Yes	Test environments, flexibility
VMDK	★★★☆☆	Limited	Yes	VMware migration
VHD/VHDX	★★☆☆☆	Limited	Yes	Hyper-V migration
ZFS ZVOL	★★★★★	Yes	Yes	Enterprise with ZFS features
Ceph RBD	★★★★★	Yes	Yes	Clusters, HA workloads

 

 

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Best Practices for Managing VM Disks

1. Match format to backend: Use RAW for block storage (LVM, ZFS, Ceph) and QCOW2 for directory storage.
2. Separate OS and data disks: Makes migration and backups easier.
3. Use thin provisioning wisely: Prevent over-allocation by monitoring disk usage.
4. Leverage snapshots for backups/testing: But avoid keeping too many long-term snapshots, as they degrade performance.
5. Migrate imported disks to RAW/ZVOL: After moving from VMware or Hyper-V, convert disks to native formats for performance.

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Conclusion

Proxmox VE’s support for multiple VM disk formats makes it versatile for both new deployments and migration scenarios. While RAW and ZVOLs deliver maximum performance for production, QCOW2 is an excellent choice for flexible lab environments. Formats like VMDK and VHDX ensure Proxmox can easily integrate into mixed virtualization environments, making transitions smoother.

By carefully choosing the right format and backend, administrators can balance performance, efficiency, and flexibility to get the most out of their Proxmox VE clusters.