— A Deeper Look at Raw, Qcow2, VMDK, and VDI
There are many different virtual machine image formats, each with its own merits, so this article will focus on a few of them and discuss their advantages and disadvantages, as well as their application scenarios.
Raw Format – Simple and Efficient
Raw format images are, as the name suggests, pure and straightforward, like a blank sheet of paper. Images in this format contain only binary data on disk, without any additional metadata or structure. Because of its intrinsic simplicity, raw mirrors are extremely portable and compatible, especially on file systems that support sparse files, and take up only as much storage space as the actual data requires. For virtualization platforms such as KVM and Xen, the raw format is often seen as the default choice because it offers near-lossless performance and easy mounting. However, it lacks advanced features such as snapshot support, and is cumbersome to manually add space and consolidate via dd commands, etc., when expansion is required.
Qcow and Qcow2 formats – Feature-Rich Upgrades
Qcow was QEMU’s early Copy-On-Write format, but has been superseded by Qcow2, which not only carries over the benefits of its predecessor, but also introduces a number of enhancements including, but not limited to, snapshot support, space-saving on file systems that do not support sparse files, optional AES encryption, and zlib compression. These enhancements make Qcow2 a popular and efficient choice, with performance comparable to that of the raw format, while offering additional management and security benefits.
VMDK Format – VMware’s Professional Standard
Developed by VMware for use in their virtualized environments, VMDK supports multiple versions such as VMware 3 & 4 or 6. This format integrates VMware’s advanced features, resulting in superior performance and functionality. While VMDK is less commonly used in KVM and Xen, on VMware ESXi, VMDK is still highly regarded for its stability and comprehensive capabilities.
VDI Format – The VirtualBox Choice
VDI is a VirtualBox-specific format designed for exchanging images on the VirtualBox platform. With the acquisition of VirtualBox by Sun Microsystems and the subsequent takeover of Sun by Oracle, VDI became part of Oracle’s technology ecosystem. Despite Oracle’s complex approach to open source, VDI continues to perform well in VirtualBox and is an excellent product for workstation-level virtualization.
Conversion and Interoperability
Converting images between different formats is a common requirement, especially for cloud service providers and enterprise data centers. Tools such as qemu-img, virt-sparsify, and virt-resize can simplify this process, making it easier to convert from raw to qcow2, or from VMDK to VDI, thus facilitating image migration and sharing between virtualized environments.
Conclusion
In summary, the choice of VM image format depends on the specific application scenario, performance needs, and requirements for advanced features. Whether it’s raw for extreme performance, qcow2 for rich functionality, or VMDK and VDI for specific virtualization platforms, they all have their own value and scope.