# Common Image Formats

IMS supports multiple image formats, but the system uses ZVHD or ZVHD2 by default.

Table 1 lists the common image formats.

Table 1 Common image formats
Image Format | Description | Remarks
--- | --- | ---
ZVHD | This format uses the ZLIB compression algorithm and supports sequential read and write. | A universal format supported by IaaS OpenStack; a format supported for imported and exported images
ZVHD2 | This format uses the ZSTD algorithm and supports lazy loading. | A format for the lazy loading feature; a format supported for imported images

QCOW2

This is a disk image supported by the QEMU simulator. It is a file that indicates a block device disk of a fixed size. Compared with the RAW format, the QCOW2 format has the following features:

- Supports a lower disk usage.
- Supports Copy-On-Write (CoW). The image file only reflects disk changes.
- Supports snapshots.
- Supports zlib compression and encryption by following Advanced Encryption Standard (AES).

A format supported for imported and exported images

VMDK | VMDK is a virtual disk format from VMware. A VMDK file represents a physical disk drive of the virtual machine file system (VMFS) on an ECS. | A format supported for imported and exported images
VHD | VHD is a virtual disk file format from Microsoft. A VHD file is a compressed file stored in the file system of the host machine. It mainly contains a file system required for starting ECSs. | A format supported for imported and exported images
VHDX | VHDX is a new VHD format introduced into Hyper-V of Windows Server 2012 by Microsoft. Compared with the VHD format, VHDX has a larger storage capacity. It provides protection against data damage during power supply failures, and the disk structure alignment has been optimized to prevent performance degradation of new physical disks in a large sector. | A format supported for imported images
RAW | A RAW file can be directly read and written by ECSs. This format does not support dynamic space expansion and has the best I/O performance. | A format supported for imported images
QCOW | QCOW manages the space allocation of an image through the secondary index table. The secondary index uses the memory cache technology and needs the query operation, which results in performance loss. The performance of QCOW is inferior to that of QCOW2, and the read and write performance is inferior to that of RAW. | A format supported for imported images
VDI | VDI is the disk image file format used by the VirtualBOX virtualization software from Oracle. It supports snapshots. | A format supported for imported images
QED | The QED format is an evolved version of the QCOW2 format. Its storage location query mode and data block size are the same as those of the QCOW2 format. However, QED implements Copy-On-Write (CoW) in a different way as it uses a dirty flag to replace the reference count table of QCOW2. | A format supported for imported images

Parent topic: Basic Concepts