Disks are block-level data storage products provided by Alibaba Cloud for ECS. They feature low latency, high performance, durability, and reliability. Disks use a distributed triplicate mechanism to ensure 99.9999999% data durability for ECS instances. This means that if service disruptions occur (for example, due to hardware failure), data within the target zone is copied to an unaffected disk to help ensure data availability.
Category of disks
As with hard disks, you can partition, format, create file systems on, and persistently store data on disks that are attached to ECS instances. The following table lists the disk categories by performance.
| Category | Description |
|---|---|
| Enhanced SSD (ESSD) | The ultra-high performance disks provided by Alibaba Cloud. ESSDs are based on the next-generation distributed block storage architecture, a 25 GE network, and remote direct memory access (RDMA) technology. Each ESSD can deliver up to 1 million of random IOPS and has low latency. For more information, see Enhanced SSDs. |
| Standard SSD | High-performance disks that feature consistent and high random IOPS performance and high data durability. |
| Ultra disk | Disks that feature high cost-effectiveness, medium random IOPS performance, and high data durability. |
| Basic disk | Disks that feature high data durability and standard random IOPS performance. |
Creation methods
A disk can be used as a system disk or a data disk based on the type of the stored data and the creation method of the disk.
| Type | Description | Create separately | Maximum number of disks per instance | Capacity |
|---|---|---|---|---|
| System disk | Shared access to a system disk is not allowed. When a system disk is initialized, the operating system type and application data are determined by the image. Only cloud disks can be used as system disks. | No. A system disk must be created with an instance. The lifecycle of the system disk is the same as that of the instance. | 1 | Depends on the image of the instance:
|
| Data disk | A data disk is often used to store application data. It can be created separately or together with an ECS instance. A disk or a Block Storage device of other types can be used as a data disk. | Yes. | 16 | A maximum of 32 TiB. For more information, see Block storage performance. |
Limits
A disk can be attached to only one ECS instance that is in the same zone as the disk. For more information, see the "Block Storage limits" section in Limits.
Billing methods
Disks can be billed on a subscription or a pay-as-you-go basis. For more information, see Subscription and Pay-as-you-go.
- Disks that are created with subscription instances or created separately for subscription instances are billed on a subscription basis.
- Disks that are created with pay-as-you-go instances or created separately for pay-as-you-go instances are billed on a pay-as-you-go basis.
After a disk is created, you can change its billing method as needed. For more information, see Convert the billing methods of cloud disks.
Related operations
You can perform the following operations on a disk as needed:
| Business requirement | Reference |
|---|---|
| Attach an idle pay-as-you-go disk to an ECS instance | |
| Purchase a subscription data disk | Create a subscription disk |
| Purchase a data disk separately | |
| Encrypt the data stored on a disk | ECS disk encryption |
| Resize a system disk | Overview |
| Resize a data disk | Overview |
| Change the operating system | Replace the system disk by using a public image |
| Back up disk data | |
| Replicate the operating system and application data of another ECS instance | Create a custom image from a snapshot |
| Restore a disk to its initial status | Reinitialize a cloud disk |
| Restore a disk to its status at a specific point in time | Roll back a disk |
| Release idle subscription disks to reduce costs | |
| Release idle pay-as-you-go disks to reduce costs |