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Overview

The Polystack Image Service supports multiple virtual disk formats. Selecting the right format affects launch performance, storage efficiency, and compatibility with your source environment. This guide compares supported formats, explains conversion workflows, and provides recommendations for common use cases.
Prerequisites
  • Basic familiarity with virtual disk concepts
  • qemu-img installed locally for format conversion (optional)

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Format Comparison

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Format Details

QCOW2 — Recommended

RAW images are unformatted disk dumps. There is zero overhead from the format layer. When Polystack Distributed Storage (Ceph RBD) is the image and volume backend, RBD handles copy-on-write natively — making RAW the optimal format.
On Ceph-backed deployments, instance launches from RAW images use zero-copy RBD clones — resulting in near-instantaneous boot times regardless of image size.
VHD and VHDX are the native disk formats for Microsoft Hyper-V. Import them directly into Polystack when migrating workloads from a Hyper-V environment.
Convert VHD/VHDX to QCOW2 after import for optimal performance and Polystack snapshot support.
VMDK is the native disk format for VMware ESXi and Workstation. Import VMDKs directly when migrating virtual machines from a VMware environment.
Convert VMDKs to QCOW2 after import to enable Polystack snapshot and resize operations.

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Convert Between Formats

Use qemu-img to convert images locally before upload:
Verify the converted image:
Verify converted image

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Next Steps

Upload an Image

Upload your converted image to the Polystack Image Service.

Image Properties

Set hardware properties matching your image format and source platform.

Image Admin Guide — Storage Backends

Configure the storage backend where images are stored after upload.

Troubleshooting

Resolve format-related launch failures and image upload errors.