Proxmox VM Disk Size Limits Explained

The maximum disk size you can assign to a VM in Proxmox depends on several factors, including the underlying storage technology, filesystem, and VM configuration. Here’s a summary of the key limitations:

1. Storage Technology Limitations

• LVM (Logical Volume Manager):
  • For 64-bit systems with Linux 2.6+ kernels, the maximum LV size is 8 Exabytes (EB)1.
  • For older 32-bit systems or 2.4 kernels, the limit is 2TB1.
• ZFS:
  • Theoretical maximum is 16 Exabytes (EB), though hardware constraints may apply27.
  • For file-based .raw volumes on ext4, the limit is 16 TiB (with 4K blocks)2.
• Ceph RBD: No inherent limits from RBD/QEMU, but practical constraints (e.g., VM disk controller type like VirtIO-SCSI) may apply. Even 2PB disks are theoretically possible2.

2. Filesystem Overhead

• ZFS RAIDZ configurations (e.g., RAIDZ1/2) incur overhead due to parity and padding. For example:
  • A 3x 3TB RAIDZ1 pool with default settings may lose ~50% of raw capacity to parity and padding, leaving ~3.6TB usable4.
  • Increasing volblocksize (e.g., to 16K) can reduce padding overhead4.

3. VM Configuration

• VirtIO-SCSI supports near-unlimited disks, but some users report limits around 30 devices per VM5. For larger setups, PCI passthrough of HBAs is recommended5.
• Disk Controllers: IDE has lower limits compared to VirtIO or SCSI. For large disks, use VirtIO-SCSI or VirtIO-block26.

4. Practical Considerations

• Proxmox GUI Defaults: The web interface may default to TiB/GiB (binary units), which can cause confusion (e.g., 32GiB ≠ 32GB)9.
• Thin Provisioning: LVM-thin or ZFS allows overallocation, but physical storage must eventually match usage89.

5. Hardware Constraints

• HBA/RAID Cards: Some adapters (e.g., Adaptec) may impose partition limits (e.g., 2TB for boot volumes)1.
• Network Storage: Performance may bottleneck on network speed (e.g., OVH’s internal network limiting striped ZFS speeds)5.

Recommendations:

• For >2TB disks, use 64-bit Proxmox, ZFS/VirtIO-SCSI, and modern kernels.
• For multi-petabyte disks, verify controller/filesystem support (e.g., ZFS or Ceph RBD)27.
• Monitor RAM requirements for ZFS (rule of thumb: 1GB RAM per 1TB storage for performance)5.

For specific setups, consult the Proxmox documentation or community forums.