• Image Management Service

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  4. Overview
  5. Basic Concepts
  6. Common Image Formats

Common Image Formats

IMS supports multiple image formats, and the system uses the ZVHD format by default.

Table 1 lists the common image formats.

Table 1 Common image formats

Image Format

Description

Remarks

ZVHD

This is a self-developed format. It 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 is a self-developed format. It uses the Zstandard (Zstd) algorithm and supports lazy loading.

A format for the lazy loading feature A format supported for imported images

QCOW2

The QCOW2 image is a type of disk image supported by the QEMU simulator. A file is used to represent a block device disk with 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 the changes of disks.
  • 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 created by VMware. A VMDK file indicates 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 provided by Microsoft. A VHD file can be compressed into a single file and stored in the file system of the host. 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 optimizes the disk structure alignment mode 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 the ECS. 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 Virtual BOX virtualization software of SUN. 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