Custom images help you run ECS instances effectively by allowing you to create multiple ECS instances with identical OS and environment data to meet scaling requirements. Custom images are based on ECS disk snapshots. You can set up identical or different configurations for ECS instances that are created from images. The illustration is as follows.



You can use a snapshot to create a custom image, including the operating system and data environment of the snapshot in the image. You can then use the custom mirror to create multiple instances with the same operating system and data environment, replicating instances easily.

You can also use an instance to create a image. See Create a custom image by using an instance.

To enhance the security of creating custom imags from snapshots, operation, Alibaba Cloud custom mirror security recommendations.

Note
  • Custom images cannot be used across regions.
  • You can change the operating system of an instance created from a custom image. The custom image can still to be used after the operating system is changed. See Change the system disk (custom image).
  • You can upgrade the instance created from a custom image, including upgrading the CPU, memory, bandwidth, and disks.
  • Custom images are independent from billing methods. Both prepaid and Pay-As-You-Go billing methods work. Custom images created from prepaid instances can used for creating Pay-As-You-Go instances. The opposite is also true.
  • If the ECS used for creating a custom image expires, or the data is erased (that is, the system disk used for the snapshot expires or is released), the custom image and the ECS instances created from the custom image will not be affected. However, auto snapshots will be cleared when an ECS instance is released.

Considerations for Linux instances

  • Do not load data disk information in the /etc/fstab file. Otherwise, instances created using this image will not start.
  • We recommend that before taking a snapshot and creating an image, umount all data disks, and then create a snapshot to create a custom image. Otherwise, ECS instances that are created based on this custom image may not start.
  • Do not upgrade the kernel or operating system version.
  • Do not change the system disk partition. The system disk only support a single root partition.
  • Check the availabe space of the the system disk to ensure that the system disk is not full.
  • Do not modify critical system files such as /sbin, /bin and /lib etc.
  • Do not modify the default logon user name root.

Procedure

  1. Log on to the ECS console.
  2. Select a region.
  3. In the left-hand navigation pane, click  Instances.
  4. Find the target instance, click the instance ID, or in the Actions column, click Manage.

  5. In the left-hand navigation pane, click Snapshots. Find the target system disk, in the Actions column, click Create Custom Image.

    The snapshot must be created from Data disks cannot be used to create custom images. 

    You can also use Snapshots > Snapshot listand select a snapshot created from a system disk to create a custom image.



  6. In the pop-up Create a custom image dialog box, do the following:
    • Confirm the snapshot ID.
    • Specify the name and description of the custom image.
    • Optional. Click Add Data Disk Snapshot to select multiple snapshots of data disks for the image. Click Add to add a data disk.
      Note

      Remove sensitive data from the data disk before creating a custom image to avoid the risk of data security.

      If the snapshot disk capacity is left blank, an empty disk will be created with the default capacity of 5 GB.

      If you select available snapshots, the disk size is the same as the size of these snapshots. 



  7. Click Create. The custom image is successfully created. In the left-side navigation pane, select  Snapshots & Images > Images to view images you have created.

FAQ for images of Linux instances

How to umount a disk and delete disk table data?

If /dev/hda5 is attached to /mnt/hda5, run any of the following three commands to detach the file system: 

umount/dev/hda5
umount/mnt/hda5
umount/dev/hda5/mnt/hda5

/Etc/fstab is an important configuration file in Linux. It contains file system details and storage devices attached at startup. If you do not want to mount a specified partition when starting the VM, delete the corresponding lines from /etc/fstab. For example, you can delete the following statement to disconnect xvdb1 upon startup: /dev/xvdb1 /leejd ext4 defaults 0 0

How to determine whether a data disk is detached and a custom image can be created?

You must ensure that the auto attach data disk statement line has been deleted from the fstab file.

Use the mount command to view information on all mounted devices. Ensure that the execution results do not contain the information of the data disk partition.

Relevant configuration files

Before creating an image, ensure the key configuration files from the following table have not been modified; otherwise, the new instance will not be able to start. As shown in the following table.

Configuration File Purpose Risks if changed
/etc/issue*, /etc/*-release, /etc/*_version For system release and version Modifying /etc/issue* will make the system release version unidentifiable, and cause instance creation failure.
/boot/grub/menu.lst, /boot/grub/grub.conf For system boot Modifying /boot/grub/menu.lst will result in kernel loading failure, and the system will not be able to start.
/etc/fstab For mounting partitions during boot. Modifying it will cause partition mounting failure, and the system will not be able to start.
/etc/shadow For storing system passwords. If this file is set to read-only, the password file cannot be edited, and instance creation will fail.
/etc/selinux/config For system security policies Modifying /etc/selinux/config and enabling SELinux will result in start failure.