You cannot decrease the sizes of system or data disks in Elastic Compute Service (ECS). You can use Alibaba Cloud Server Migration Center (SMC) to decrease the sizes of disks.
Prerequisites
The preparations for the migration are completed. For more information, see Before you begin.
Background information
SMC is designed to balance the cloud-based and offline workloads of Alibaba Cloud users. You can also use SMC to decrease the sizes of ECS disks.
SMC allows you to create a custom image from an ECS instance. When you create the custom image, you can reset the size of a disk to a smaller value. The same procedure and limits apply when you use SMC to decrease the size of a disk and migrate data. However, when you decrease the size of a disk, the associated ECS instance is used as the migration source. When you decrease the size of a disk, data is migrated between ECS instances. Data migration between ECS instances is more efficient and has a lower error rate.
Take note that when you use SMC to decrease the size of a disk, some attributes of
the ECS instance used as the migration source are changed, such as the instance ID
(InstanceId) and public IP address. If your instance resides in a VPC, you can change the public
IP address to an elastic IP address (EIP). This way, you can retain the public IP
address. We recommend that you use SMC to decrease the size of a disk when instances
are associated with EIPs, or when you are less dependent on public IP addresses.