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

If I have activated OSS, are snapshots automatically saved to my existing OSS buckets?

No, snapshots are not automatically saved to existing OSS buckets. Snapshots are stored independently from your OSS buckets. You do not need to create OSS buckets for snapshots.

Can a custom image that was created from a snapshot be saved to an OSS bucket?

Yes, you can export a custom image to your OSS bucket for future download. For more information, see Export a custom image. However, custom images cannot be saved directly to OSS buckets.

How is the storage fee for snapshots calculated?

Snapshots are billed monthly on a pay-as-you-go basis. The price per GiB for snapshot storage is the same as that for the OSS Standard storage class. For more information about snapshot prices in different Alibaba Cloud regions, see the Pricing tab on the Elastic Compute Service product page.

For more information about examples of how to use the pay-as-you-go billing method, see Snapshots.

Where can I view snapshot prices in different Alibaba Cloud regions?

Snapshots are billed on a monthly basis. The price per GiB for snapshot storage is the same as that for the OSS Standard storage class. For more information about snapshot prices in different Alibaba Cloud regions, see the Pricing tab on the Elastic Compute Service product page. Scroll down to the Snapshot tab. You can view the snapshot prices by region in the Snapshot service fee details section of the Snapshot tab. Download price

What impact do overdue payments have on my stored snapshots?

The snapshot feature is disabled within 24 hours after your payment becomes overdue. The length of time that your payment becomes overdue determines whether snapshots are retained. For more information, see Overdue payments.

I use snapshots frequently. How can I reduce the amount of fees for snapshots?

We recommend that youmaintain a manageable number of snapshots and delete snapshots that are no longer needed. For more information, see Reduce snapshot fees.

Does the snapshot feature provide a free tier?

No, you must pay for the snapshot feature. You are charged for snapshots after the first snapshot is created. For more information, see Snapshots.

Do automatic snapshots differ from or conflict with manual snapshots?

No, both manual and automatic snapshots are data files that capture the point-in-time status of a disk. However, when an automatic snapshot is being created for a disk, you cannot create a manual snapshot for the disk. You must wait until the automatic snapshot is created.

Can I create snapshots for local disks?

No, snapshots cannot be created 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.

I have created a snapshot for an encrypted data disk and generated an image, but I cannot share the image. Why?

To ensure data privacy, custom images created from encrypted snapshots cannot be shared. We recommend that you use unencrypted snapshots to create custom images so that you can share the images with other users.

How is the size of the first full snapshot of a disk calculated?

The first snapshot created for a disk is a full snapshot. This snapshot contains all the data stored on the disk at the time when you create the snapshot. Therefore, the snapshot size is equal to the used capacity of the disk. For example, if the capacity of a disk is 200 GiB and 122 GiB has been used, the first full snapshot of the disk is 122 GiB in size. For more information, see Incremental snapshots and View the snapshot size.

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

No, no storage space is released. When you delete files from an ECS instance, only delete tags are added to the headers of the files, but the used storage space of the disks is not reduced.

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

  • Problem description: After you delete files from an instance and create a snapshot for the instance, you find that the size of the snapshot is not reduced or is larger than the disk capacity displayed in the file system.
  • Cause analysis: When a snapshot is being created, the system identifies empty blocks to reduce the size of the snapshot. However, some empty blocks are filled when you format the file system, delete files, or write data to the blocks. Therefore, the snapshot size may be larger than the disk capacity that is displayed in the file system. Possible causes:
    • File system metadata occupies disk space.
    • Logical Block Addressing (LBA) is used to divide disks into blocks that have the same size. When the file system is initialized, data is written into a large number of blocks. This write operation also occupies disk space.
    • To reduce resource consumption, the file system only adds delete tags to the headers of files instead of actually 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. Therefore, the size of the snapshot is larger than the disk capacity 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 which data blocks in the LBA range are no longer considered to be in use and therefore can be erased. As a result, 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 take the form of 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.Relationship between a file system and a disk or a snapshot
Note The preceding figure indicates that if data has been written to data blocks, the blocks are recorded in the snapshot regardless of whether the related files have been deleted from the disks. In the file system, when you delete files, only tags are added to the headers of the files, but the disk space used by the files is not released on the disks.

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

  • Manual snapshots are never deleted by Alibaba Cloud regardless of whether the corresponding disks or instances have been released.
  • To prevent automatic snapshots from being deleted, you can modify the corresponding automatic snapshot policy by setting Keep Snapshots to Always Keep. After the maximum number of automatic snapshots for a disk is reached, the earliest snapshot is deleted when a new snapshot is created. For more information, see Modify an automatic snapshot policy. For more information about the snapshot quota, see Limits.

How can I delete snapshots to reduce backup costs?

  • Manual snapshots: You can manually delete manual snapshots.
  • Automatic snapshots: You can manually delete automatic snapshots. In addition, when the maximum number of automatic snapshots is reached, the earliest snapshot is deleted when a new snapshot is created.

An automatic snapshot is created for a disk of an instance. Will the snapshot be deleted after the system disk of the instance is changed, the instance expires, or the disk is released?

  • If Delete Automatic Snapshots While Releasing Disk is selected for the automatic snapshot policy, automatic snapshots are deleted when the corresponding instance or disk is released.
  • If Delete Automatic Snapshots While Releasing Disk is not selected for the automatic snapshot policy, automatic snapshots are retained for the period of time specified by the automatic snapshot policy. You can modify the automatic snapshot policy. For more information, see Modify an automatic snapshot policy.

How can I delete snapshots that have been used to create images and disks?

  • You can delete snapshots that have been used to create disks. After a snapshot is deleted, operations that require the original snapshot data, such as re-initializing the system disk cannot be performed. For more information, see Reinitialize a system disk.
  • If a snapshot has been used to create custom images, you must delete those custom images before you can delete the snapshot.
  • You can delete images that have been used to create instances. After an image is deleted, operations that require the original snapshot data, such as re-initializing the system disk cannot be performed. For more information, see Reinitialize a system disk.

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

No, the automatic snapshot policy can be executed.

Can I create multiple automatic snapshot policies for a disk?

No, you cannot create multiple automatic snapshot policies for a disk.

How can 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 was accidentally released. You can also create snapshots to recover from network attacks, change operating systems, or provide data support for a production environment. If an error occurs, you can use snapshots to roll back disks to reduce risks. For more information, see Create a snapshot for a disk and Roll back a disk by using a snapshot.

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

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

I created Instance A in the China (Hangzhou) region and created snapshots for 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 the Instance A after Instance A is released. If you want to use disk snapshots to restore an instance, you must 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 attach the disks to Instance B. For more information, see Create a disk from a snapshot and Attach a data disk.

Why cannot snapshots of an instance be used to roll back the disks of the instance?

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

What are the differences and relationship between snapshots and images?

Differences between snapshots and images:
  • Images can be used directly to create ECS instances, but snapshots cannot.
  • Images can be used to restore instance data across regions, but snapshots cannot. For more information, see Copy custom images.
  • A snapshot can be a data backup of the system disk or a data disk of an ECS instance, whereas an image must contain the system disk data of an ECS instance.
  • Snapshots can be used to back up data stored on disks. Images can be used to create one or more ECS instances.
Relationship between snapshots and images:
  • When you create a custom image from an instance, ECS creates a snapshot for each disk of the instance. The created custom image contains the snapshots of all the disks of this ECS instance. For more information, see Create a custom image from an instance.
  • You can also create custom images from system disk snapshots. For more information, see Create a custom image from a snapshot.

How do I migrate snapshots from one account to another?

Snapshots cannot be migrated across accounts. You can create an image from the snapshot that you want to migrate and share the image with another account. For more information, see Create a custom image from a snapshot and Share or unshare a custom image.

To migrate a data disk snapshot from Account A to Account B, perform the following operations:
  1. Create an image from the original instance to which the data disk is attached. For more information, see Create a custom image from a snapshot.
  2. Share the image to Account B. For more information, see Share or unshare a custom image.
  3. Within Account B, use the image to create a pay-as-you-go instance. For more information, see Create an ECS instance by using a custom image.
  4. Create a snapshot for a data disk of the new instance. For more information, see Create a snapshot for a disk.
  5. Release the new instance. For more information, see Release an instance.

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

No, data disk snapshots cannot be used to create custom images. Only system disk snapshots can be used to create custom images.

Can I download or export ECS instance snapshots to my on-premises device?

No, snapshots cannot be downloaded or exported to on-premises devices. You can create images from snapshots and then export the images. For more information, see Create a custom image from a snapshot and Export a custom image. You can also use the OOS template named ACS-ECS-BulkyCreateAndExportImage to create multiple custom images at a time and export the images to an OSS bucket in a specific region. Then, you can download the custom images in the OSS console.

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

Your snapshot was used to create a custom image. You must delete the custom image before you can delete the snapshot. For more information, see Delete a custom image.