This topic provides answers to commonly asked questions about ECS snapshots.

If I have activated Object Storage Service (OSS), will snapshots be automatically saved to my OSS buckets?

No, snapshots are not automatically saved to existing OSS buckets. Snapshots are independently stored from your OSS buckets. You do not need to create new 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 to download in the future. For more information, see Export a custom image. However, custom images cannot be directly saved to an OSS bucket.

How is the storage fee for snapshots calculated?

Snapshots are billed 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. The pay-as-you-go billing is charged on a monthly basis. For information about snapshot prices of various Alibaba Cloud regions, see the Pricing tab on the Elastic Compute Service page.

For more information about examples of the pay-as-you-go billing method, see Snapshot billing.

Where can I view a list of snapshot prices by Alibaba Cloud region?

The price per GiB for snapshot storage is the same as that for the OSS Standard storage class. The pay-as-you-go billing is charged on a monthly basis. For more information about snapshot prices of various Alibaba Cloud regions, see the Pricing tab on the Elastic Compute Service page. Scroll down to the Snapshot Fee section to view the price list based on the regions. You can also download the list of snapshot prices based on Alibaba Cloud regions in the CSV or JSON format by clicking Download price.Download price

What impact will overdue payments have on my stored snapshots?

The snapshot service is suspended 24 hours after your account payments become overdue. After your account has overdue payments:

  • In the first 15 days, all existing snapshots are retained, and no automatic snapshots can be created. All automatic snapshots whose retention period is less than 15 days are deleted.
  • After 15 days, all snapshots are deleted, except for those that have been used to create disks or custom images. The automatic snapshot policy is also deleted.

I often use snapshots. How can I reduce the amount of fees incurred?

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

Is there a free tier for the snapshot service?

No, you need to pay for the snapshot service. Snapshots start to incur fees after you create your first snapshot. For more information, see Snapshot billing.

Do automatic snapshots differ from or conflict with manual snapshots?

No, both manual and automatic snapshots are data files of a disk at a specific point in time. However, you cannot create manual snapshots while automatic snapshots are being created. You must wait until the automatic snapshots have been created.

Can I create snapshots for local disks?

No. If you want to improve the availability of applications, we recommend that you use data redundancy at the application layer or create deployment sets for clusters.

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. This allows you to share these images with other users.

After an enhanced SSD (ESSD) is released, will its local snapshots be deleted?

Manual snapshots are never deleted by Alibaba Cloud regardless of whether the corresponding disk or instance has been released.

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 the snapshot is created. 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 disk snapshot is 122 GiB in size. For more information, see Incremental snapshots and View the snapshot size.

Will deleting files from an ECS instance release storage space?

No, no storage space is released. When you delete files from an ECS instance, the space is not cleared on the disks. In this case, only tags are added to the headers of the deleted files.

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

  • Issue: Assume that you have created a snapshot after you delete files from the ECS instance. The size of the snapshot has not been reduced or is larger than the disk capacity displayed in the file system.
  • Cause: After snapshots are created, a mechanism identifies empty blocks to reduce the size of the snapshot. However, these empty blocks are filled when you format the file system, delete files, or write data to blocks. Therefore, the snapshot size may be larger than the disk capacity that is currently displayed in the file system. Specific causes:
    • File system metadata occupies disk space.
    • During file system initialization, data is written into a large number of blocks that are divided equally from the logical block address (LBA). This write operation also occupies a certain amount of disk space.
    • To improve performance, the file system only adds tags to the headers of files when the files are deleted. The disk cannot detect the deleting instruction. Therefore, the deleted data blocks are still allocated and copied to snapshots.
    • The KVM virtio block and Xen block front drivers do not support the TRIM instruction (indicates that data in the LBA is no longer in use and can be deleted). As a result, the disk cannot determine whether 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 as required. This process is how snapshots are created. 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 In the preceding figure, all data blocks with data written to them are recorded in the snapshot. This applies even if the related files have been deleted from the disk. In the file system, only tags are added to the headers of files to be deleted but space is not cleared 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 disk or instance has been released.
  • To prevent automatic snapshots from being deleted, you can set the Keep Snapshots parameter to Always Keep when you modify the automatic snapshot policy. Then, only the earliest snapshots are deleted when the quota limit of the snapshots is reached. 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. When the maximum number of snapshots has been reached, the system also automatically deletes the earliest automatic snapshot.

Are automatic snapshots deleted after the system disk 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, the retention period specified by the automatic snapshot policy applies. You can modify the automatic snapshot policy as required. 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, you cannot perform operations based on the status of the original snapshot data, such as the operation to re-initialize the system disk. For more information, see Reinitialize a system disk.
  • If the 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, you cannot perform operations based on the status of the original snapshot data, such as the operation to re-initialize the system disk. For more information, see Reinitialize a system disk.

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

No, the automatic snapshot policy will not fail to 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 avoid losing data due to incorrect operations?

You can create snapshots to back up data before you perform high-risk operations. For example, you can create a snapshot if you need to modify critical system files, migrate instances from the classic network to a VPC, back up data, or restore an instance that was released by mistake. You can also create a snapshot to prevent against network attacks, change operating systems, or provide data support for a production environment. If an error occurs, you can roll back the disk to reduce risks. For more information, see Create a normal snapshot and Roll back a disk by using a snapshot.

After I change the system disk, 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 an instance in China (Hangzhou) and created snapshots for data disks of the instance. I purchased another instance in China (Hangzhou) after the previous one expired and was released. Can I use the snapshots from my original instance to restore the instance?

No, if you want to use snapshots to restore an original instance, you must make sure that the instance from which the snapshots were created is not released. You can use one of snapshots of the original data disk to create a disk and attach the new disk to the new instance. For more information, see Create a disk from a snapshot and Attach a data disk.

Why am I unable to use a snapshot to roll back a disk of an ECS instance?

You can check the snapshot based on the returned error message. For more information about the common issues and solutions regarding snapshots to roll back disks, see Block Storage FAQ.

What are the differences and relationships between snapshots and images?

The following content lists the differences between snapshots and images:
  • Images can be directly used 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 either the system disk or a data disk of an ECS instance. However, an image must contain the system disk data of an ECS instance.
  • You can use snapshots to back up data on a disk, and use images to create one or more ECS instances.
The following content lists the relationships between snapshots and images:

How do I migrate snapshots from one account to another?

Snapshots cannot be migrated. 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 custom images.

To migrate a data disk snapshot from Account A to Account B, follow these steps:
  1. Create an image from the original instance of the data disk snapshot. 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 custom images.
  3. In 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 from a data disk of the newly created instance. For more information, see Create a normal snapshot.
  5. Release the newly created instance. For more information, see Release an instance.

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

No, the snapshot that is used to create a custom image must be a snapshot of a system disk.

Can I download or export ECS instance snapshots to a local device?

No, snapshots cannot be downloaded or exported to a local device. You can create an image from a system disk snapshot and then export the image. For more information, see Create a custom image from a snapshot and Export a custom image. You can also use the Operation Orchestration Service (OOS) template named ACS-ECS-BulkyCreateAndExportImage to create multiple custom images at a time and export the images to an OSS bucket. Then, you can download the custom images in the OSS console. If you want to perform the OOS task in the China (Hangzhou) region, click ACS-ECS-BulkyCreateAndExportImage.

Why does the system prompt "RequestId: xxx" is associated 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 custom images.