Snapshot

What is Snapshot?

A snapshot is a point-in-time capture of data status in a volume. Before you perform a business-sensitive operation, you can schedule snapshot creation at specified time points to record the state of the root volume, data volume, or memory of a VM instance. This allows rollback in case of breakdowns. If you want to backup data for a long term, you can use the Backup Service.

Snapshot Category

Snapshots can be classified into the following two categories:

Manual snapshot: snapshots that are manually created for root volumes or data volumes.
Auto snapshot: snapshots that are automatically created at a scheduled time interval or in specific scenarios.
- To create auto snapshots for a resource at a scheduled time interval, you need to create a scheduled snapshotting policy for the resource. This way, the system creates snapshots at scheduled time points for the resource.
- The system may create an auto snapshot in the following scenarios:
  - Root volume capacity expansion
  - Data volume capacity expansion
  - VM reset
  - Snapshot rollback
    Note:
    - By default, the system does not create a snapshot in the snapshot rollback scenarios.
    - You can enable or disable one-time snapshot-creation in snapshot rollback scenarios on the Global Setting page.
      Choose Setting > Platform Setting > Global Setting > Advanced > Resource Center > Resource Pool > Snapshot. Turn on or off the Auto Create Volume Snapshot switch according to your business needs.
  - Image creation of root volume or data volume on Ceph primary storage

Snapshot View

ZStack Cloud provides the following two views of snapshots:

List view: In this view, snapshot information are displayed in a list, including their name, type, size, creation time, and supported operations.
Topology view: In this view, snapshots are displayed in a hierarchical tree with snapshot chains and snapshot nodes. You can locate the current snapshot, reset canvas, zoom in or out the canvas, and enter the full screen of the view.

Scenarios

Snapshots can be used for the following scenarios:

Fast recovery from breakdowns: If the production environment breaks down, you can rollback snapshots to return to the normal state. Snapshot rollback is a temporary use for unexpected breakdowns. To backup data for a long term, we recommend that you use Backup Service.
Data exploration: You can create snapshots for production data and use the snapshots to do data mining, query, and development and test.
Fault tolerance improvement: Before you perform a business-sensitive operation such as upgrading your system or migrating data, we recommend that you create one or more snapshots. This allows fast recovery to the normal state in case of any errors or exceptions during the operation.

Snapshot Fundamentals

Non-Ceph Storage QCOW2 External Snapshot Mechanism

For non-Ceph storage, including LocalStorage, NFS, Shared Mount Point, and Shared Block, ZStack Cloud uses QCOW2 (QEMU Copy On Write, Version 2) external snapshot mechanism.

Snapshot Tree and Snapshot Chain
A volume corresponds to a snapshot chain. ZStack Cloud allows you to create a snapshot tree for a volume. Each branch of the snapshot tree is a snapshot chain.

图 1. Snapshot Tree
A snapshot tree includes the following information:
- Snapshot chain: a chain of snapshots of a volume that are associated with each other. Each branch of a snapshot tree is a snapshot chain.
- Snapshot node: a node in a snapshot chain that represents a snapshot of a volume.
- Snapshot size: the storage space that a snapshot occupies. You can view the total size of all snapshots in a snapshot tree and the size of an individual snapshot node.
Note:
- For non-Ceph storage, a snapshot node can have a maximum of 128 nodes by default. You can customize the maximum length of a snapshot chain Global Setting by setting Maximum Incremental Volume Snapshot. For Ceph storage, a snapshot node can have a maximum of 32 nodes, including the auto-created and manually-created nodes.
- When the length of a snapshot chain reaches the threshold:
  - If an auto-snapshot is created, the system automatically deletes the snapshot that was created at the earliest time.
  - If you manually create a snapshot, you need to manually delete the snapshot that you do not need.
- In the production environment, we recommend that you limit the number of snapshots created for a volume to 5. Too many snapshots will lower VM and volume performance, increase data security risks, and occupy storage space of primary storage. To back up data for the long term, you can use Backup Service.
Create Snapshot
When an external snapshot is created, an empty qcow2 file is created. The backing file of the empty file points to the previous qcow2 file. This qcow2 file is set to read-only and become a snapshot. Data is written to the empty qcow2 file.
- Create single-chain snapshots based on the backing file.
  
  图 2. Create Single-Chain Snapshots
  Suppose you have a raw image. You use the image to create a VM instance named VM 1 and then create snapshot 1A and 1B for VM 1.
  - Raw image: a disk image file that includes a complete operating system and boot process. The image is a read-only file used as the base file.
  - VM 1: creates an empty file named Overlay-1. The backing file of the empty file points to the base file. The base file is kept read-only and becomes a snapshot. Data is written to Overlay-1.
  - Snapshot 1A: creates an empty file named Overlay-1A. The backing file of the empty file points to Overlay-1. Overlay-1 is set to read-only and becomes a snapshot. Data is written to Overlay-1A.
  - Snapshot 1B: creates an empty file named Overlay-1B. The backing file of the empty file points to Overlay-1A. Overlay-1A is set to read-only and becomes a snapshot. Data is written to Overlay-1B. Overlay-1B is the last snapshot in the snapshot chain and is in Active state. VM 1 uses the disk file that corresponds to the snapshot Overlay-1B.
- Create a multi-chain snapshot chain based on the backing file.
  
  图 3. Create Multi-Chain Snapshots
  Suppose you have a raw image. You use the image to create VM instances named VM 1, VM 2, and VM 3 and then create snapshot 1A and 1B for VM 1, snapshot 2A for VM 2, and snapshot 3A for VM 3.
  - Raw image: a disk image file that includes a complete operating system and boot process. The image is a read-only file used as the base file.
  - Snapshot chain 1:
    - VM 1: creates an empty file named Overlay-1. The backing file of the empty file points to the base file. The base file is kept read-only and becomes a snapshot. Data is written to Overlay-1.
    - Snapshot 1A: creates an empty file named Overlay-1A. The backing file of the empty file points to Overlay-1. Overlay-1 is set to read-only and becomes a snapshot. Data is written to Overlay-1A.
    - Snapshot 1B: creates an empty file named Overlay-1B. The backing file of the empty file points to Overlay-1A. Overlay-1A is set to read-only and becomes a snapshot. Data is written to Overlay-1B. Overlay-1B is the last snapshot in the snapshot chain and is in Active state. VM 1 uses the disk file that corresponds to the snapshot Overlay-1B.
  - Snapshot chain 2:
    - VM 2: creates an empty file named Overlay-2. The backing file of the empty file points to the base file. The base file is kept read-only and becomes a snapshot. Data is written to Overlay-2.
    - Snapshot 2A: creates an empty file named Overlay-2A. The backing file of the empty file points to Overlay-2. Overlay-2 is set to read-only and becomes a snapshot. Data is written to Overlay-2A. Overlay-2A is the last snapshot in the snapshot chain and is in Active state. VM 1 uses the disk file that corresponds to the snapshot Overlay-2A.
  - Snapshot chain 3:
    - VM 3: creates an empty file named Overlay-3. The backing file of the empty file points to the base file. The base file is kept read-only and becomes a snapshot. Data is written to Overlay-3.
    - Snapshot 3A: creates an empty file named Overlay-3A. The backing file of the empty file points to Overlay-3. Overlay-3 is set to read-only and becomes a snapshot. Data is written to Overlay-3A. Overlay-3A is the last snapshot in the snapshot chain and is in Active state. VM 1 uses the disk file that corresponds to the snapshot Overlay-3A.
Consolidate Snapshots
External snapshots are interdependent with each overlay snapshot depending on its backing file. Each snapshot captures certain amounts of data. You cannot directly delete a snapshot to shorten a snapshot chain. To shorten a snapshot chain, you can blockcommit or blockpull external snapshots.
- Blockcommit
  Consolidate overlay snapshots of a snapshot chain into backing files.
  
  图 4. Blockcommit
  
  Suppose you have a raw image. You use the image to create VM instance named VM 1 then create three dependent snapshots named 1A, 1B, and 1C for VM 1. Then you blockcommit snapshot 1A and 1B to VM 1. This makes the backing file of snapshot 1C which is in Active state points to VM 1 and shortens the snapshot chain. Snapshot 1A and 1B are no longer useful and you can delete the two snapshots.
- Blockpull
  Consolidate backing files of a snapshot chain into overlay snapshots.
  
  图 5. Blockpull
  
  Suppose you have a raw image. You use the image to create VM instance named VM 1 then create three dependent snapshots named 1A, 1B, and 1C for VM 1. Then you blockpull snapshot 1A and 1B to snapshot 1C of VM 1. This makes the backing file of snapshot 1C which is in Active state points to VM 1 and shortens the snapshot chain. Snapshot 1A and 1B are no longer useful and you can delete the two snapshots.

Ceph Storage Snapshot Mechanism

For Ceph storage, ZStack Cloud uses the mainstream ROW (Redirect-On-Write) snapshot mechanism of Ceph.

Create a Snapshot

Creating a snapshot is categorized into the following scenarios:

Create VM snapshots:
- Single snapshot: creates a snapshot for the root volume of a VM instance.
- Snapshot group: creates snapshots for the root volume, data volumes, and memory of a VM instance.
Create data volume snapshots: creates snapshots for data volumes.
Create elastic baremetal instance snapshots: create snapshots for an elastic baremetal instance.

Create VM Snapshot

On the main menu of ZStack Cloud, choose Resource Center > Resource Pool > Virtual Resource > VM Instance. On the VM Instance page, select a running, stopped, or paused VM instance, and then choose Actions > Snapshot and Backup > Create Snapshot. Then, the Create Snapshot for VM Instance dialog box appears.

On the dialog box, set the following parameters:

Name: Enter a name for the volume.
The name must be 1 to 128 characters in length and can contain Chinese characters, letters, digits, spaces, hyphens (-), underscores (_), periods (.), parenthesis (), colons (:), and plus signs (+) and cannot begin or end with spaces.
Description: Optional. Enter a description for the snapshot.
Snapshot Type: Select either of the following type of snapshots that you want to create:
- Single Volume Snapshot: creates a snapshot for the root volume of the VM instance.
- Snapshot Group: creates a group of snapshots for the volumes of the VM instance, including the root volume and data volumes.
  Note:
  - You cannot create snapshot groups for a VM instance to which shared volumes are attached.
  - Before you can create a memory snapshot, make sure that the VM instance is in the running state.
  - Before you can create a memory snapshot, detach the peripheral devices (if any) from the VM instance first.
  - When you create a memory snapshot, to ensure memory consistency, the VM instance will be in the paused state for seconds.
  - You cannot create a memory snapshot if the VM is running on a Ceph or Vhost primary storage. If a VM has a memory snapshot, it cannot be migrated to a Ceph or Vhost primary storage.

You can also create snapshots for a VM instance on its details page. On the details page, click the Snapshot tab. On the tab, click Create Snapshot. Then, the Create Snapshot for VM Instance dialog box appears. On the dialog box, set the required parameters and create a snapshot for the VM instance.

Create Data Volume Snapshot

On the main menu of ZStack Cloud, choose Resource Center > Resource Pool > Virtual Resource > Volume > Data Volume. On the Data Volume page, select a data volume, and then choose Actions > Create Data Volume Snapshot. Then, the Create Data Volume Snapshot dialog box appears.

On the dialog box, set the following parameters:

Name: Enter a name for the volume.
The name must be 1 to 128 characters in length and can contain Chinese characters, letters, digits, spaces, hyphens (-), underscores (_), periods (.), parenthesis (), colons (:), and plus signs (+) and cannot begin or end with spaces.
Description: Optional. Enter a description for the snapshot.

You can also create snapshots for a data volume on its details page. On the details page, click the Snapshot tab. On the tab, click Create Snapshot. Then, the Create Data Volume Snapshot dialog box appears. On the dialog box, set the required parameters and create a snapshot for the volume.

Create Elastic Baremetal Instance Snapshot

On the main menu of ZStack Cloud, choose Resource Center > Elastic Baremetal Management > Elastic Baremetal Instance. On the Elastic Baremetal Management page, locate an elastic baremetal instance. Click Actions > Create Snapshot. Then, the Create Elastic Baremetal Instance Snapshot page is displayed.

On the displayed page, set the following parameters:

Name: Enter a name for the snapshot. The name must be 1 to 128 characters in length and can contain Chinese characters, letters, digits, spaces, hyphens (-), underscores (_), periods (.), parenthesis (), colons (:), and plus signs (+) and cannot begin or end with spaces.
Description: Optional. Enter a description for the snapshot.
Type: You can only create a snapshot for the elastic baremetal instance root volume.

You can also enter the details page of an elastic baremetal instance. On the details page, choose Snapshot tab and click Create Snapshot. Then, the Create Elastic Baremetal Snapshot is displayed.

Notes

You can create a snapshot for a running or stopped instances.
You cannot create a snapshot group for an elastic baremetal instance.
You cannot create a snapshot group for a VM instance attached with shared volumes.
In the production environment, we recommend that you create a maximum of five snapshots for a volume. Excessive number of snapshots lowers I/O performance of volumes or VM instances, increases data security risks, and occupies storage space of primary storage. To back up data for the long term, you can use Backup Service.
In the production environment, we do not recommend that you create snapshots for VM instances with high I/O operations. This is because if a VM instance has high I/O operations, some data may be still in the memory and is not stored on the volume when you create a snapshot for the VM instance. This part of data is not reserved in the snapshot, thus affecting data integrity.
If the VM/volume resides on Shared Block primary storage (thick provisioning), make sure that the available space on the primary storage is greater than the volume size before you create the snapshot.
You cannot create a snapshot for a shared volume that is created on a Shared Block or Ceph primary storage.

Manage a Snapshot

On the main menu of ZStack Cloud, choose Resource Center > Resource Pool > Resource Service > Snapshot. Then, the Snapshot page is displayed.

Manage a VM Snapshot

Action	Description
Create VM Snapshot	Create a snapshot for a running or a stopped VM instance. VM snapshots are categorized into single snapshots and snapshot groups: Single snapshot: a snapshot based on the VM root volume. Snapshot group: snapshots created for both the VM root volume and the data volumes attached to the VM instance. You can revert snapshots in a snapshot group uniformly. Note: You cannot create a snapshot group for a VM instance attached with shared volumes. If the VM/volume resides on Shared Block primary storage (thick provisioning), make sure that the available space on the primary storage is greater than the volume size before you create the snapshot.
Edit VM Snapshot	Edit the name and description of a VM snapshot.
Delete VM Snapshot	Delete a VM snapshot no longer required. Note: Snapshots created on a LocalStorage, NFS, NFS, or Shared Block primary storage are stored as a tree structure. Deleting a root snapshot also deletes snapshots and unbinds snapshot groups on its sub branches. Please exercise caution. Snapshots created on a Ceph primary storage are stored respectively instead of as a tree structure. Deleting a snapshot does not affect other snapshots. When you delete a snapshot in the tree structure, the system automatically figures out and deletes its associated snapshots. Deleting a snapshot group also deletes data volume snapshots in the group and snapshots on its sub branches. Please exercise caution. Deleting snapshots in bulk also deletes the snapshots and unbinds the snapshot groups on their sub branches. Please exercise caution. Before a snapshot on a LocalStorage, NFS, NFS, or Shared Block primary storage is deleted, the system merges the snapshot data to corresponding volumes first. This action occupies a certain capacity of the primary storage. If the primary storage capacity is insufficient, you might fail to delete the snapshot. Deleting a snapshot occupies I/O. Do not restart the management node service during this process. You cannot delete snapshot groups which contain memory snapshots in bulk. You can delete a snapshot group containing a memory snapshot separately. You cannot delete snapshots and snapshot groups at the same time. You cannot delete a snapshot if it is used by linked clone VM instances. To delete the snapshot, flatten relevant resources and try again.
Revert VM Snapshot	Recover the VM instance to the state at the time point when you created the snapshot. Note: With a snapshot, you can recover the VM root volume to the state at the time point when you created the snapshot. With a snapshot group, you can recover both the VM root volume and data volumes to states at the time point when you created the snapshot group. Make sure that the VM instance is stopped before you revert the snapshot. Reverting a snapshot occupies primary storage capacity. If the system predicts that the revert operation may cause the primary storage usage to exceed 90% or exceed the Global Setting Primary Storage Usage Threshold, it intercepts this operation. You can set whether to make the VM instance start automatically after the snapshot reversion. If a VM snapshot cannot be reverted, check the following points: One or more data volumes snapshots in a snapshot group is deleted. Then, you cannot revert the snapshot group, but can only revert snapshots in it respectively. One or more data volumes attached to the VM instance are deleted. Then, you cannot revert the snapshot group, but can only revert snapshots in it respectively. One or more data volumes are detached from the VM instance. Then, you cannot revert the snapshot group, but can only revert snapshots in it respectively. The VM instance is attached with a new data volume. Then, you cannot revert the snapshot group, but can only revert snapshots in it respectively.
Unbind Snapshot Group	Restore the VM snapshot to a single snapshot and disassociated the VM instance from the data volume snapshots. This action is irreversible. Please exercise caution.

Manage a Data Volume Snapshot

Action	Description
Create Data Volume Snapshot	Create a snapshot for a data volume. Note: If the volume resides on Shared Block primary storage (thick provisioning), make sure that the available space on the primary storage is greater than the volume size before you create the snapshot. You cannot create a snapshot for a shared volume on a Shared Block primary storage.
Edit Name and Description	Edit the name and description of a data volume snapshot.
Revert Volume Snapshot	Recover the volume data to the state at the time point when you create the snapshot. Note: Stop the VM instance the volume attached to or detach the data volume from the VM instance before you revert the volume snapshot. Reverting a snapshot occupies primary storage capacity. If the system predicts that the revert operation may cause the primary storage usage to exceed 90% or exceed the Global Setting Primary Storage Usage Threshold, it intercepts this operation.
Delete Volume Snapshot	Delete a data volume snapshot no longer required. You can delete volume snapshots in bulk. Note: Snapshots created on a LocalStorage, NFS, NFS, or Shared Block primary storage are stored as a tree structure. Deleting a root snapshot also deletes snapshots on its sub branches. Please exercise caution. Snapshots created on a Ceph primary storage are stored respectively instead of as a tree structure. Deleting a snapshot does not affect other snapshots. When you delete a snapshot in the tree structure, the system automatically figures out and deletes its associated snapshots. If the snapshot to be deleted is in a snapshot, deleting this snapshot also deletes snapshots on its sub branches. The action is irreversible. Please exercise caution. If you delete a root volume snapshot in a snapshot group, the group is unbound and other snapshots in the group become single snapshots. If you delete a data volume snapshot in a snapshot group, the group is not unbound but other snapshots in the group can be reverted only as single snapshots. Before a snapshot on a LocalStorage, NFS, NFS, or Shared Block primary storage is deleted, the system merges the snapshot data to corresponding volumes first. This action occupies a certain capacity of the primary storage. If the primary storage capacity is insufficient, you might fail to delete the snapshot. You cannot delete snapshot groups in bulk. You cannot delete snapshots and snapshot groups at the same time. You cannot delete a snapshot if it is used by linked clone volumes. To delete the snapshot, flatten relevant resources and try again.

Manage an Elastic Baremetal Instance Snapshot

Action	Description
Create Elastic Baremetal Instance Snapshot	Create a snapshot for a running or stopped elastic baremetal instance. Note: If the instance resides on Shared Block primary storage (thick provisioning), make sure that the available space on the primary storage is greater than its volume size before you create the snapshot.
Edit Elastic Baremetal Instance Snapshot	Edit the name and description of an elastic baremetal instance snapshot.
Revert Elastic Baremetal Instance Snapshot	Recover the elastic baremetal instance data to the state at the time point when you create the snapshot. Note: You can only recover the root volume data of the elastic baremetal instance to the state at the time point when you create the snapshot. You can revert the snapshot only for a stopped elastic baremetal instance. Before you revert a snapshot, stop the corresponding elastic baremetal instance. Reverting a snapshot occupies primary storage capacity. If the system predicts that the revert operation may cause the primary storage usage to exceed 90% or exceed the Global Setting Primary Storage Usage Threshold, it intercepts this operation. You can choose whether to make the elastic baremetal instance start automatically after the snapshot is reverted.
Delete Elastic Baremetal Instance Snapshot	Delete an elastic baremetal instance snapshot. Note: Snapshots created on a LocalStorage, NFS, NFS, or Shared Block primary storage are stored as a tree structure. Deleting a root snapshot also deletes snapshots on its sub branches. Please exercise caution. Snapshots created on a Ceph primary storage are stored respectively instead of as a tree structure. Deleting a snapshot does not affect other snapshots. When you delete a snapshot in the tree structure, the system automatically figures out and deletes its associated snapshots. Before a snapshot on a LocalStorage, NFS, NFS, or Shared Block primary storage is deleted, the system merges the snapshot data to corresponding volumes first. This action occupies a certain capacity of the primary storage. If the primary storage capacity is insufficient, you might fail to delete the snapshot. Deleting a snapshot occupies I/O. Do not restart the management node service during this process.

Snapshot Usage Suggestion

This topic provides some suggestions for you to better use snapshots.

Suggestions for Creating Snapshots

We recommend that you create less than 5 snapshots in a production environment. Too many snapshots negatively affect the VM/volume I/O performance, reduce the data security level, and occupy the primary storage space. For a long-term backup, we recommend that you use Backup Management.
For the data integrity, we recommend that you do not create a snapshot for a high-I/O VM instance. If you create a snapshot for the VM instance when it is exercising a high-I/O operation, you may fail to record the data that has not been stored by volumes to the snapshot.
To ensure successful reversion of snapshots, we recommend that you do not create snapshot groups with memory snapshots for a VM instance that has had ordinary snapshot groups (groups without memory snapshots), and vice versa.
We recommend that you do not schedule automatic snapshot creations frequently in a production environment. Frequent scheduled snapshots negatively affect the I/O performance, storage space, and business stability of a production environment.
- The snapshots created by frequent scheduled jobs affect VM business. For example, a VM instance on a Ceph primary storage reduces its write performance, and a VM instance on a LocalStorage or shared primary storage reduces its read performance if you create too many snapshots for it.
- The snapshots created by frequent scheduled jobs affects the I/O output of a primary storage.
- The snapshots created by frequent scheduled jobs occupy a large space on a primary storage.

Automatic Scheduled Snapshots

The system automatically create snapshots in the following scenarios:

Resizing a VM root volume.
Resizing a data volume.
Reimaging a VM instance.
Creating images for volumes on Ceph primary storage.
Using a snapshot to make a data rollback.
Note:
- By default, the system does not automatically create snapshots when you exercise snapshot rollbacks.
- To make the system automatically create snapshots when you exercise snapshot rollbacks, follow these method:
  On the main menu of ZStack Cloud, choose Settings > Platform Setting > Global Setting > Advanced > Resource Center > Resource Pool > Snapshot. Enable Volume Snapshot Auto Creation.
- We recommend that you create a snapshot for the volume to record its current data state before you make the snapshot rollback.

Snapshot Impacts

System Capacity Occupation
- In a LocalStorage or shared storage scenario, an incremental snapshot occupies only a small space, while a full snapshot occupies twice as much space.
- In a Ceph primary storage scenario, creating a snapshot does not occupy a storage space, but the Copy-On-Write triggered when you write in the disk may consume a space as much as the disk size.
Business Impacts
- Creating Snapshot: In normal scenarios, creating a snapshot does not affect the business. If you create a full snapshot, it brings an excess storage network I/O. In a LocalStorage or shared storage scenarios, it occupies a certain disk I/O bandwidth, but brings little influence the disk IOPS.
- Deleting Snapshot: In normal conditions, deleting a snapshot also deletes the snapshots on its sub branches and merges the data to the disk. This brings an excess disk I/O, and slightly slows down the business I/O.

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