Bug Reference

CLOUDSTACK-10066

Branch

masterMaster, patches will be submitted through to the review board.

Introduction

Purpose

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Currently CloudStack supports VM snapshots for VmWareVMware. This feature will add the support for VM Snapshots in Hyper-V.

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VM Snapshot - A Hyper-V snapshot is an encapsulation of a running VM’s state, data, and hardware configuration

Feature Specifications

VM Snapshot creation

• VM snapshots form a tree structure, each VM snapshot can have one(or zero) parent snapshot.
• A current snapshot refers to the most recent snapshot compared to the current state of the VM (although a domain might have snapshots without a current snapshot if snapshots have been deleted in the meantime)
• Two types of snapshots: disk, which takes a snapshot of all disks of specified VM; disk and memory, which takes CPU/memory snapshot in addition to disks snapshot.

• Support disk snapshot when specified VM is in running and stopped state
• Support disk and memory snapshot when specified VM is in running state

VM Snapshot limitations

• Detaching/attaching VM volume is not allowed if there are VM snapshots because any changes to the disk layout will break the semantics of VM-based snapshot
• VM's memory snapshots will be automatically discarded if VM's service offering is upgraded.
• VM snapshot operations and volume snapshot operations can not be performed concurrently.
• For one VM, only one VM snapshot operation is allowed at a time. (no concurrent operations)
• Customers should only use CS to take a snapshot. CS maintains the tree in database, out-of-band snapshots will not be tracked or sync to CS
• Limit per account not supported
• Recurring snapshot not supported

VM Snapshot deletion

• Deleting a snapshot should not have any impact to its subsequent snapshots
• Snapshots will be destroyed when VM is destroyed

VM Snapshot revert

• Revert VM from running/stopped to a disk+memory snapshot, result in running state
• Revert VM from running/stopped to a disk snapshot, result in stopped state

VM Snapshot List

• Can list with commonly used parameters, like vmId, account, domainId, state..etc
• Support query by keyword (unimplemented)

Performance consideration

• Both create and revert should be completed on the scale of seconds
• As the number snapshots for one VM grows, performance may downgrade. Users should have the awareness to control the length of VM snapshot chain.

Use cases

• Create snapshot for a specified VM
• Revert VM to a specified snapshot
• Delete a specified snapshot
• List snapshots for a specified VM

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• Add snapshot action and [view snaptshots] in VM detail page

         Image Added

        Image Removed

• Snapshots List:

       Image Removed

• VM snapshot detail

         Image Added       

• VM snapshot detail

          Image Added       

HighLevel WorkFlow

VMSnapshot state machine

Image Added

createVMSnapshot:

Common workflow

1. Check authority, concurrency, existence.
2. Allocate VM snapshot entry in DB.
3. Transit the VM and VM snapshot state to snapshotting/creating.
4. Prepare TO object and CreateVMSnapshotCommand.
5. Send the command to the agent.
6. Update DB, like current/parent fields or volume table, depending on CreateVMSnapshotAnswer and TO object.
7. Transit VM and VM snapshot state.

revertToVMSnapshot:

Common workflow

1. Check authority, concurrency, existence.
2. Call advanceStart or advanceStop first if revert will change VM's state; for example, when reverting a stopped VM to a DiskAndMemory snapshot, we will start this VM first and then revert it.
3. Transit the VM and VM snapshot state to reverting.
4. Prepare TO objects and send command.
5. Update DB with information from Answer object.
6. Transit VM and VM snapshot state.

deleteVMSnapshot:

Unlike VM expunging, VM snapshot deletion is designed as a sync operation, there is no daemon thread scanning and expunging them.

The implementation is fairly straightforward:

1. Transit VM snapshot to expunging state.
2. Prepare TO object and send command.
3. Update snapshots tree.
4. Mark as removed.

VMSnapshotSync:

1. Add VM snapshot sync to fullSync and fullHostSync.
2. It will check if there is any VM snapshot in transient states.
3. The transient state found during host connection usually means mgmt server restart/outrage, or hypervisor cluster down. Because mgmt server has no idea if those tasks succeed or not, it will re-send the command in question

Enable/disable on a per hypervisor :

Add enable/disable by hypervisor_capabilities,

Add a new column ` vm_snapshot_enabled` in table `hypervisor_capabilities`, and change related VO/Dao

Set vm_snapshot_enabled = 1

Check hypervisor_capabilities when createVMSnapshot

Testing

Suggest following (but not limited) basic test scenarios

Create one VM snapshot with snapshotMemory (on, off) when VM is (running, stopped)

Revert to the previous snapshot when VM is (running, stopped)

Create multiple VM snapshot with snapshot memory (on, off, mixed) when VM is (running, stopped), the snapshots should form a tree hierarchy, such as:

    A

  /    \

B     C

Revert to any snapshots in the tree when VM is (running, stopped)

Delete (current, any, all) VM snapshots

Attach/detach a volume to a VM when this VM has VM snapshots.

Upgrade VM service offering when VM has snapshots with snapshot memory (on, off)

take Volume Snapshot when associated VM has VM snapshots

        Image Removed

Important

Do not delete .avhd files directly from the storage location.

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Important questions:

When we stop VM from cloudstackCloudStack, on Hyper-V that VM is destroyed. When VM gets destroyed all the associated VM snapshots also get destroyed.

To Overcome overcome this we can export the VM which also contains the snapshots information.

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1. Export VM when we are stopping the VM.
   
   Pros
   
   1.  In In this case we  , we have to perform the costly export operation only when it is needed(when we cannot avoid).
   2. VM snapshots will be fast as then it needs to keep only differential data.
      
      
   Cons
   1. The stop operation will get affected, which we can make asynchronous and export VM only when the VM snapshots has have been taken on that VM.
   2. What will be done if VM is started before the export operation has been completed?
   3. What will happen if primary goes down in middle of an export operation or export operation has not yet started?
      1. whenever host or primary comes up first export the VM then delete it, if it has snapshots
      2. What state of that VM will be returned in hostVMstatereport for vmsync?
         
         
2. Export VM at every snapshot
   
   Pros
   1. Probability The probability of stop operation being affected will be low. Will only be affected when stop operation occurs in the process of exporting VM.
   
   Cons
   1. VM snapshots will be slow

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