Alibaba Cloud snapshots are complete, read-only copies of disk data at certain points in time. They are an effective solution to disaster recovery and often used to back up data, create custom images, and implement disaster recovery.

Use scenarios

You can use snapshots for the following scenarios:
  • Disaster recovery and backup: You can create a snapshot for a disk, and then use the snapshot to create another disk to implement zone- or geo-disaster recovery.
  • Environment clone: You can use a system disk snapshot to create a custom image, and then use the custom image to create an ECS instance to clone the environment.
  • Data development: Snapshots can provide near-real-time production data for applications such as data mining, report queries, and development and tests.
  • Enhanced fault tolerance: You can roll a disk back to a previous point in time by using a snapshot to reduce the risk of data loss caused by unexpected occurrence. You can create snapshots in the following two cases:
    • Create snapshots on a regular basis to prevent losses caused by unexpected occurrences. These unexpected occurrences include: writing incorrect data to disks, accidently releasing ECS instances, data errors caused by application errors, and data loss due to hacking attempts.
    • Create a snapshot before you perform high-risk O&M operations, such as changing operating systems, upgrading applications, and migrating business data.

Snapshot classification

There are two types of snapshots: normal and local snapshots. Snapshots are classified based on how they are stored.
Note Local snapshots are during public preview as of January 17, 2020.
The following table describes the classification of snapshots based on different standards.
Classification standard Type Description Use scenario
Storage method Normal snapshot

A snapshot is stored in an OSS instance that resides in the same region as the source disk or Shared Block Storage device. You can create normal snapshots for disks and Shared Block Storage devices. Normal snapshots can be created for system disks and data disks.

Normal snapshots are ideal for scenarios that require high disaster recovery capabilities. However, it takes a long time to create normal snapshots.
Local snapshot

A local snapshot is stored in the same storage cluster as the source disk and can be used to perform data backup and disk rollback within a few seconds. Local snapshots can be created only for enhanced SSDs (ESSDs).

  • You can use local snapshots to quickly back up key business systems that contain huge data, such as databases, containers, and SAP HANA.
  • You can also use local snapshots to back up data before you perform high-risk operations to reduce the waiting time for backups. High-risk operations include changing system disks, resizing disks, and updating system patches.
  • You can use local snapshots in DevOps applications to accelerate the creation of custom images and the startup of ECS instances.
Creation method Manual snapshot

A manual snapshot is a snapshot that you manually create.

 You can create a manual snapshot before performing high-risk operations to enhance fault tolerance.
Automatic snapshot
An automatic snapshot is a snapshot that is created automatically based on an automatic snapshot policy. You create and apply an automatic snapshot policy to a disk. ECS then creates snapshots automatically for the disk at specified points in time.
Note All automatic snapshots are normal snapshots.
You can use automatic snapshots to back up data and improve the security of business data.
Creation order Full snapshot

A full snapshot is the first snapshot created for a disk. This snapshot contains all the data stored on the disk at the time the snapshot is created.

N/A
Incremental snapshots

Incremental snapshots are all snapshots created after the full snapshot. These snapshots contain only incremental data blocks.

N/A
Encryption Encrypted snapshot An encrypted snapshot is a snapshot that is created from an encrypted disk.
Note All encrypted snapshots are normal snapshots.
You can create encrypted snapshots if your business is required to comply with certain security standards.
Unencrypted snapshot An unencrypted snapshot is a snapshot that is created from an unencrypted disk. N/A

Pricing

Snapshots are billed based on their sizes. They can be billed on a pay-as-you-go basis. For more information, see Snapshot billing methods.

Limits

For the limits and quotas of snapshots, see the "Snapshot limits" section in Limits.

Local snapshots have the following limits:
  • Local snapshots can be created only for ESSDs.
  • A maximum of 10 local snapshots can be retained for an ESSD.
  • When you use a local snapshot to create a disk, the specified disk size cannot be larger than the snapshot size.
  • You cannot use automatic snapshot policies to create local snapshots.
  • Local snapshots cannot be created for encrypted disks, Shared Block Storage devices, and local disks.

Benefits

Compared with the snapshot feature of traditional storage services, Alibaba Cloud ECS snapshots have the following benefits:
Item Alibaba Cloud ECS snapshot Snapshot of traditional storage service
Capacity Unlimited capacity for storing large amounts of business data. Limited capacity based on the capacity of the storage device.
Scalability Auto scaling allows you to extend storage devices to any size within seconds. Low scalability due to limited storage performance, available capacity, and vendor support.
Total cost of ownership (TCO) Pay only for the storage space occupied by your snapshots. Pay large upfront costs for software licenses, reserved storage space, upgrade, and maintenance.
Security Support data encryption. If you encrypt your source disk, all snapshots created for the disk are encrypted. Unencrypted snapshots cannot be directly converted to encrypted snapshots, and encrypted snapshots cannot be directly converted to unencrypted snapshots. For more information, see ECS disk encryption. Encryption attributes and policies are subject to the underlying storage logic. If the storage architecture has a security flaw, snapshots created based on this architecture may not be secure.
Impact on performance Redirect-on-write (ROW)
  • Reduce impacts on the I/O performance of the source disk.
  • Snapshots do not affect service availability and can be created at any time without affecting user experience.
Typically copy-on-write (COW), but may also ROW or other methods. COW negatively affects the data write capabilities of the source system.

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