Snapshot FAQ

Updated at: 2025-01-24 14:25

This topic provides answers to some frequently asked questions about Elastic Compute Service (ECS) snapshots.

Are snapshots automatically saved to the OSS buckets that I created after I activated OSS?

Snapshots are automatically stored in Alibaba Cloud Object Storage Service (OSS). For data security reasons, you cannot select the OSS buckets that you created to store snapshot data.

  • By default, snapshot data is stored in zone-redundant storage (ZRS) buckets in regions where OSS supports ZRS to maximize data redundancy.

  • In regions where OSS does not support ZRS, snapshot data is stored in locally redundant storage (LRS) buckets. If your business requires high availability, we recommend that you copy snapshots to back up data across regions.

Can I save a custom image that is created from a snapshot to an OSS bucket?

You cannot directly save custom images to OSS buckets. However, you can export a custom image that is created from a snapshot to an OSS bucket and download the custom image from the OSS bucket for use. For more information, see Export a custom image.

What are the differences between OSS resource plans and SCUs?

  • OSS resource plans: subscription resource plans, including storage plans and data transfer plans. For more information, see Resource plans.

  • Storage capacity units (SCUs): subscription resource plans that can be used to offset capacity fees for various storage resources, such as cloud disks, snapshots, and OSS resources. After you purchase an SCU in a region, the SCU is automatically applied to offset pay-as-you-go fees for eligible cloud disks and snapshots in the region. If the capacity of the cloud disks and snapshots exceeds the capacity of the SCU, you are automatically charged for the overage at the pay-as-you-go rates. For more information, see SCUs.

How do I restore the data of an ECS instance from snapshots after the instance is released?

To restore the data of an ECS instance from snapshots after the instance is released, perform the following steps:

  1. Create a custom image from the snapshots that were created for the disks attached to the ECS instance. For more information, see Create a custom image from a snapshot.

  2. Create an ECS instance from the custom image. For more information, see Create an instance by using a custom image.

  3. Connect to the new ECS instance, check whether the data of the released ECS instance is completely restored, and then copy or further process the data.

    Important

    You are charged for the computing and storage resources of the new ECS instance. To prevent unnecessary fees, we recommend that you release the new ECS instance at the earliest opportunity after you restore the data of the released ECS instance.

What billing method is used for snapshots?

Snapshot fees may include snapshot storage fees and data transfer fees for snapshot replication. By default, you are charged snapshot fees based on the pay-as-you-go billing method. You can use SCUs to offset the bills of snapshot storage fees. SCUs cannot be used to offset the bills of data transfer fees for snapshot replication. For more information, see Snapshots.

How do I view the pricing of snapshots for different Alibaba Cloud regions?

Visit the Elastic Compute Service product page and click the Pricing tab. Then, scroll down to and click the Snapshot tab to view the unit price of snapshots in each region.

image

Does the Snapshot service have a free quota?

Alibaba Cloud provides a free quota of 5 GB for snapshot storage. You are charged for snapshot storage that exceeds the quota. For information about snapshot billing, see Snapshots.

Am I charged for activating the Snapshot service?

No, you are not charged for activating the Snapshot service. After you create snapshots, you are charged for the storage space occupied by the created snapshots.

Am I charged for using automatic snapshot policies?

No, you are not charged for using automatic snapshot policies. However, you are charged for the automatic snapshots created based on automatic snapshot policies. Both automatic snapshots and manual snapshots incur charges. For more information, see Snapshots.

Am I charged for using snapshot-consistent groups?

No, you are not charged for using snapshot-consistent groups. However, you are charged for snapshots in snapshot-consistent groups based on the storage space that is occupied by the snapshots. For more information, see Snapshots.

What are the differences between manual snapshots and automatic snapshots?

Automatic and manual snapshots are copies of disk data at specific points in time. Automatic and manual snapshots have the following differences:

  • Different creation methods

    • Manual snapshots are manually created. You can create snapshots for cloud disks on the Snapshots page or on the Cloud Disk tab of the Block Storage page and configure snapshot parameters based on your business requirements. For more information, see Create a snapshot.

    • Automatic snapshots are created based on automatic snapshot policies. After you create an automatic snapshot policy and apply the policy to a cloud disk, the system automatically creates snapshots for the disk based on the points in time or interval specified in the policy. For more information, see Create an automatic snapshot policy.

  • Different values in the Creation Method column

    When you view manual and automatic snapshots in the snapshot list, the following differences exist:

    • The Creation Method value of an automatic snapshot is Automatic Snapshot.

    • The Creation Method value of a manual snapshot is Manual Snapshot.

  • Different snapshot names

    • The name of a manual snapshot is specified when the snapshot is created.

    • The name of an automatic snapshot starts with auto2.0. The name is in the auto2.0_yyyyMMdd_SnapshotPolicyId format.

      • auto2.0: the prefix of the name of an automatic snapshot.

      • yyyyMMdd: the date on which the snapshot is created. yyyy indicates the year, MM indicates the month, and dd indicates the day.

      • SnapshotPolicyId: the ID of the automatic snapshot policy based on which the snapshot is created.

Note

When you use manual and automatic snapshots, take note of the following limits:

  • If a snapshot is being manually or automatically created for a cloud disk when the point in time specified in the associated automatic snapshot policy is reached, the automatic snapshot cannot be created based on the policy at the point in time. An automatic snapshot is created for the disk at the next point in time specified in the automatic snapshot policy.

  • If an automatic snapshot is being created for a cloud disk, you cannot create a manual snapshot for the disk. Wait until the automatic snapshot is created.

Can I create snapshots for local disks?

No, you cannot create snapshots for local disks. If you want to improve the availability of applications, we recommend that you implement data redundancy at the application layer or create deployment sets for instances. For more information, see Local disks.

Is the storage space released when I delete files from an ECS instance?

No, the storage space is not released when you delete files from an ECS instance. When you delete files from an ECS instance, only delete tags are added to the headers of the files. The storage space on cloud disks that is occupied by the files is not reduced.

Why is the size of a snapshot larger than the disk capacity that is displayed in the file system?

  • Problem description: After you delete files from an ECS instance and create a snapshot for the instance, the size of the snapshot may not be reduced or may be larger than the disk capacity that is displayed in the file system.

  • Cause: When a snapshot is being created, the system identifies empty blocks to reduce the size of the snapshot. However, specific empty blocks are occupied due to various operations, such as the operations performed to format the file system, delete files, or write data. In this case, the snapshot size may be larger than the disk capacity that is displayed in the file system. This issue may occur due to one of the following reasons:

    • File system metadata occupies disk space.

    • Logical Block Addressing (LBA) is used to split disks into blocks that have the same sizes. When the file system is initialized, data is written to a large number of blocks. The write operation also occupies disk space.

    • To reduce performance consumption, the file system only adds delete tags to the headers of files instead of deleting the data when you delete the files. The disk cannot detect the delete operation, and the system allocates the data blocks and copies the blocks to the snapshot. In this case, the size of the snapshot exceeds the disk capacity that is displayed in the file system.

    • Kernel-based Virtual Machine (KVM) virtio block and Xen block front modules do not support the TRIM command. The TRIM command informs a disk of which data blocks in the LBA range are no longer used and can be deleted. In this case, the disk cannot determine whether the data can be deleted.

What is the relationship between a file system and a disk or a snapshot?

You can create a file system in a disk partition. The file system manages disk space. These management tasks are converted into I/O requests in the disk. The disk records the status of data blocks and copies data to OSS buckets. Snapshots are created in this process. The following figure shows the relationship between a file system and a snapshot.

image
Note

The preceding figure shows that if data is written to data blocks, the snapshot records the blocks regardless of whether the related files are deleted from the disk. In the file system, only tags are added to the headers of the files when you delete files, but the disk space used by the files is not released on the disk.

How do I prevent snapshots from being deleted by Alibaba Cloud?

  • Whether a snapshot is deleted is determined by the retention period of the snapshot.

    When you create a snapshot or an automatic snapshot policy, you can set the Retention Period parameter to Permanently (Until Deleted) or Until Deleted or specify a retention period. After the retention period of a snapshot ends, the system automatically deletes the snapshot. You can extend the retention period of a manual or automatic snapshot based on your business requirements. For more information, see Extend the retention period of a snapshot.

  • If you enable the Delete Automatic Snapshots While Releasing Disk feature for a cloud disk, when the disk is released, whether manually, along with the instance, or by replacing the system disk, even if the automatic snapshots have not expired, the automatic snapshots will also be deleted in advance along with the disk. If you disable the Delete Automatic Snapshots While Releasing Disk feature for a cloud disk, automatic snapshots created from the disk are not released together with the disk. For more information, see Enable or disable Delete Automatic Snapshots While Releasing Disk.

A snapshot is created for a cloud disk of an instance. Will the snapshot be deleted after the cloud disk is released in the system disk replacement, instance release, or cloud disk release scenario?

Scenario

System disk replacement

Instance release

Cloud disk release

Scenario

System disk replacement

Instance release

Cloud disk release

Manual snapshot

The manual snapshots of a cloud disk are not deleted when the disk is released.

Automatic snapshot

  • If you enable the Delete Automatic Snapshots While Releasing Disk feature for the original system disk, the automatic snapshots created from the original system disk are released when the original system disk is released.

  • If you disable the Delete Automatic Snapshots While Releasing Disk feature for the original system disk, whether the automatic snapshots created from the original system disk are released is determined by the retention period specified in the automatic snapshot policy associated with the original system disk.

  • The associated automatic snapshot policy is disassociated from the original system disk. You can configure an automatic snapshot policy for the new system disk.

If you enable both the Release Disk with Instance and Delete Automatic Snapshots While Releasing Disk features for a cloud disk, the disk is released when the associated instance is released and the automatic snapshots created from the disk are released together with the disk.

  • If you enable the Delete Automatic Snapshots While Releasing Disk feature for a cloud disk, the automatic snapshots created from the disk are released when the disk is released.

  • If you disable the Delete Automatic Snapshots While Releasing Disk feature for a cloud disk, whether the automatic snapshots created from the disk are released is determined by the retention period specified in the automatic snapshot policy associated with the disk.

How do I delete snapshots that were used to create images and cloud disks?

  • You can forcefully delete snapshots that were used to create cloud disks. After you delete a snapshot, you cannot perform operations that require the original snapshot data, such as re-initializing the system disk. For more information, see Re-initialize a system disk.

  • To delete snapshots that were used to create custom images, you must first delete the custom images.

For more information, see Delete a snapshot.

If I used an automatic snapshot created based on an automatic snapshot policy to create a custom image or a cloud disk, does the automatic snapshot policy fail to be executed?

No, the automatic snapshot policy can be executed. After you apply an automatic snapshot policy to cloud disks, Alibaba Cloud automatically creates snapshots for the disks at the points in time or interval specified in the policy. Creation of a custom image or cloud disk from an automatic snapshot does not affect the execution of the automatic snapshot policy.

Can I configure multiple automatic snapshot policies for a cloud disk?

No, you can configure only one automatic snapshot policy for each cloud disk.

If I set the Retention Period parameter to Until Deleted when I create an automatic snapshot policy, can I delete the snapshots created based on the policy?

Yes, you can delete snapshots created based on the policy.

  • After you confirm that you no long require a snapshot, you can delete the snapshot. For information about how to delete a snapshot, see Delete a snapshot.

  • You can also enable the Delete Automatic Snapshots While Releasing Disk feature for a cloud disk to automatically release the automatic snapshots created from the disk when the disk is released.

How do I prevent data loss caused by accidental operations?

You can create snapshots to back up data before you perform high-risk operations. For example, you can create snapshots if you want to modify critical system files, migrate instances from the classic network to a virtual private cloud (VPC), back up data, or restore an instance that is accidentally released. You can also create snapshots to recover from network attacks, replace operating systems, or provide data support for a production environment. If an error occurs, you can use snapshots to roll back cloud disks to reduce risks. For more information, see Create a snapshot and Roll back a disk by using a snapshot.

After I replace the system disk of an instance, can I use a snapshot of the original system disk to roll back the new system disk?

No, you cannot use a snapshot of the original system disk to roll back the new system disk.

Note

You can use the snapshots to create pay-as-you-go disks and then attach the disks to ECS instances for data restoration. After data is restored, we recommend that you release the disks as soon as possible. For more information, see Create a disk from a snapshot, Attach a data disk, and Release a disk.

I created Instance A in the China (Hangzhou) region and created snapshots for the data disks of Instance A. After Instance A expired and was released, I created Instance B in the China (Hangzhou) region. Can I use the data disk snapshots of Instance A to restore Instance A?

No, you cannot use the data disk snapshots of Instance A to restore Instance A after Instance A is released. If you want to use disk snapshots to restore an instance, make sure that the instance for which the snapshots are created is not released. To restore the data of Instance A, you can use the data disk snapshots of Instance A to create disks and then attach the disks to Instance B. For more information, see Create a disk from a snapshot and Attach a data disk.

Why am I unable to use snapshots of an instance to roll back the disks of the instance?

You can check the snapshots based on the returned error message. For information about the common disk rollback issues and the corresponding solutions, see FAQ.

What are the differences and connections between snapshots and images?

Differences between images and snapshots

Difference

Image

Snapshot

Difference

Image

Snapshot

Definition

You can create an image from an ECS instance. The image is a full backup of the ECS instance and contains the configurations, operating system, installed software, and data of the instance. For more information, see the Use an image to back up and restore data section of the "ECS disaster recovery solutions" topic.

You can create a snapshot for a disk or a snapshot-consistent group for a set of disks to create a point-in-time backup of disk data. For more information, see the Use a snapshot to back up and restore data section of the "ECS disaster recovery solutions" topic.

Supported scenario

  • Back up systems that do not change in a short term, such as released or updated application systems. Create ECS instances. For example, you can use a custom image to create an ECS instance that hosts multiple applications.

  • Migrate systems and data. For example, you can migrate ECS instances from the classic network to virtual private clouds (VPCs).

    Restore systems across regions or zones. For example, you can use a custom image to create an ECS instance in a different zone, or copy a custom image to a different region and then use the image copy to create an ECS instance in that region.

  • Back up disk data on a regular basis.

    You can use automatic snapshot policies to automatically create snapshots of disks to back up disk data on a daily, weekly, or monthly basis.

  • Temporarily back up disk data. Examples:

    • Create snapshots to back up system data before a temporary system change, such as system update or application release.

    • Create a snapshot to back up disk data before you resize a system disk.

    • Create a snapshot for a disk and then create a new disk from the snapshot to migrate the disk data to the new disk.

Functionality

  • Images can be used to create ECS instances.

  • An image only contains the system disk data of an ECS instance.

  • Images can be used to create ECS instances or replace the system disks of ECS instances.

  • Snapshots cannot be used to create ECS instances.

  • A snapshot can be a data backup of the system disk or a data disk of an ECS instance.

  • Snapshots can be used to restore data only on the disks from which the snapshots are created.

Connections between images and snapshots

  • When you create a custom image, take note of the following items:

    • You can create a custom image from a system disk snapshot. For more information, see Create a custom image from a snapshot.

    • When you create a custom image from an ECS instance, a snapshot is created for the system disk and each data disk on the instance. Then, all created snapshots constitute a custom image. For more information, see Create a custom image from an instance.

  • When you delete custom images and snapshots, take note of the following items:

    • When you delete a custom image, you can choose to delete or retain the associated snapshots. For more information, see Delete a custom image.

    • When you delete a snapshot, the system checks whether the snapshot is associated with an image. If the snapshot is associated with an image, you must delete the image before you can delete the snapshot. For more information, see Delete a snapshot.

How do I migrate snapshots from one account to another account?

Snapshots cannot be migrated across accounts. You can share the snapshots of one account with another account. For more information, see Share a snapshot.

Can I use a data disk snapshot to create a custom image?

No, you cannot use data disk snapshots to create custom images. You can use only system disk snapshots to create custom images.

Can I download or export snapshots to on-premises devices?

No, you cannot download or export snapshots to on-premises devices. You can create custom images from snapshots and then export the custom images to your on-premises device.

Note

Specific limits apply when you export custom images. Before you export the custom images created from snapshots, check whether the images meet the requirements that are described in the Limits section of the "Export a custom image" topic.

Why does the "RequestId: xxx" error message appear when I delete a snapshot in a snapshot chain?

The message appears because your snapshot was used to create a custom image. Before you can delete the snapshot, you must delete the custom image. For more information, see Delete a custom image.

How do I view the data stored in a snapshot?

You cannot directly view the data stored in a snapshot. To view the data stored in a snapshot, you can use one of the following methods:

  • On this page (1)
  • Are snapshots automatically saved to the OSS buckets that I created after I activated OSS?
  • Can I save a custom image that is created from a snapshot to an OSS bucket?
  • What are the differences between OSS resource plans and SCUs?
  • How do I restore the data of an ECS instance from snapshots after the instance is released?
  • What billing method is used for snapshots?
  • How do I view the pricing of snapshots for different Alibaba Cloud regions?
  • Does the Snapshot service have a free quota?
  • Am I charged for activating the Snapshot service?
  • Am I charged for using automatic snapshot policies?
  • Am I charged for using snapshot-consistent groups?
  • What are the differences between manual snapshots and automatic snapshots?
  • Can I create snapshots for local disks?
  • Is the storage space released when I delete files from an ECS instance?
  • Why is the size of a snapshot larger than the disk capacity that is displayed in the file system?
  • What is the relationship between a file system and a disk or a snapshot?
  • How do I prevent snapshots from being deleted by Alibaba Cloud?
  • A snapshot is created for a cloud disk of an instance. Will the snapshot be deleted after the cloud disk is released in the system disk replacement, instance release, or cloud disk release scenario?
  • How do I delete snapshots that were used to create images and cloud disks?
  • If I used an automatic snapshot created based on an automatic snapshot policy to create a custom image or a cloud disk, does the automatic snapshot policy fail to be executed?
  • Can I configure multiple automatic snapshot policies for a cloud disk?
  • If I set the Retention Period parameter to Until Deleted when I create an automatic snapshot policy, can I delete the snapshots created based on the policy?
  • How do I prevent data loss caused by accidental operations?
  • After I replace the system disk of an instance, can I use a snapshot of the original system disk to roll back the new system disk?
  • I created Instance A in the China (Hangzhou) region and created snapshots for the data disks of Instance A. After Instance A expired and was released, I created Instance B in the China (Hangzhou) region. Can I use the data disk snapshots of Instance A to restore Instance A?
  • Why am I unable to use snapshots of an instance to roll back the disks of the instance?
  • What are the differences and connections between snapshots and images?
  • Differences between images and snapshots
  • How do I migrate snapshots from one account to another account?
  • Can I use a data disk snapshot to create a custom image?
  • Can I download or export snapshots to on-premises devices?
  • Why does the "RequestId: xxx" error message appear when I delete a snapshot in a snapshot chain?
  • How do I view the data stored in a snapshot?
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