How Can I Attach a Snapshot-based System Disk to an ECS as Its Data Disk?

Scenarios

To restore data, a system disk snapshot of ECS A is used to create disk A. Then, disk A is attached to ECS B as a data disk. However, after ECS B is restarted, data disk A may be attached as the root file system but not the system disk of ECS B. In such a case, the data of ECS A is contained in the root file system of ECS B.

Possible Causes

When Linux ECSs start, the root file systems to be mounted are identified by disk label but not UUID. However, system disk labels are the same, which may lead to a system disk attachment error.

Solution

Replace the /dev/disk/by-label/ROOT files in /etc/fstab with the files in /dev/disk/by-id. The new files start with a unique virtio-EVS-ID.

To do so, perform the following operations:

1. Check whether the root file system is identified by disk label. If information similar to the following is displayed in the configuration files in /etc/fstab of the target ECS, its root file system is identified by disk label:

   /dev/disk/by-label/ROOT  /  ext4 defaults 1 1

2. After attaching data disk A to ECS B, run the following command to obtain the disk ID:

   $ ls -l /dev/disk/by-id/

   total 0
   lrwxrwxrwx. 1 root root 9 Nov 16 19:40 virtio-20211065-0f57-46d4-9 -> ../../vda
   lrwxrwxrwx. 1 root root 10 Nov 16 19:40 virtio-20211065-0f57-46d4-9-part1 -> ../../vda1
   lrwxrwxrwx. 1 root root 9 Nov 16 19:40 virtio-842dbfd3-9f2c-4273-9 -> ../../vdb
   lrwxrwxrwx. 1 root root 10 Nov 16 19:40 virtio-842dbfd3-9f2c-4273-9-part1 -> ../../vdb1

3. Run the following command to change the disk ID to the desired one in the configuration files in /etc/fstab of ECS B:
   vi /etc/fstab

   /dev/disk/by-id/virtio-20211065-0f57-46d4-9-part1 / ext4 defaults 1 1
   /dev/disk/by-id/virtio-842dbfd3-9f2c-4273-9-part1 /vdb1-test ext4 defaults 1 1

4. Press Esc to exit editing mode.
5. Run the following command to save the configuration and exit:

   :wq

6. Restart the ECS for the modification to take effect.