# Live Recovery of Virtual Machines for Hyper-V

Live recovery quickly restores and starts a VM, before the full restore completes. Live recovery is especially useful when you need to quickly put a failed VM back into production, and when you need to validate that a backup is usable for disaster recovery.

## Considerations

- You can perform live recovery operations using streaming backups and backup copies.

- The software uses a 3DFS cache on the MediaAgent to perform the live recovery operation. By default, the 3DFS cache is located in the Job Results folder for the VSA (access node). During a Hyper-V live recovery operation, the software creates a network share that exists until the live recovery operation completes.

- The 3DFS cache folder permissions are used as the access permissions for this network share. If all users have full permissions on the 3DFS cache folder, then everyone can access the network share. To make the network share more secure, specify a different path for the 3DFS cache folder using the s3dfsRootDir additional setting and use an ACL to restrict user access to the network share.

  Note

  For Hyper-V restore operations, changing the disk provisioning type of VHD disks during restore is not supported. The restored disk must use the same provisioning type (Fixed or Dynamic) as the source disk. Converting between Fixed and Dynamic disk types during restore is not supported and may result in restore failures or conflicts.

## Live Recovery Process

Data is restored from the backup as needed to enable the operations requested by the VM, and the full restore completes as resources allow. The backup is not modified by the restore process.

The process for a Live Recovery includes the following steps:

1. The 3DFS server (VSA proxy) creates the UNC path, which contains the VHDX file and configuration file.

2. The 3DFS server registers the UNC path with the Callback File System (CBFS).

3. The 3DFS server fetches disk related information from indexing, and then, creates disks on the UNC path. The Disk data is restored on demand.

4. The 3DFS server fetches the network information, and then, creates the new network.

5. The VM is imported.

6. The VM is started (unless you move the Power on VMs during restore toggle key to the left to turn it off).

7. Read and write requests are passed to the 3DFS server layer through the CBFS.

8. If you enter a value (other than 0) in the Delay migration (in hours) box, the delay is completed.

9. The storage is moved from the 3DFS cache to the VM.

   During this move, writes to the VM are managed by the Hyper-V Virtual Machine Management service (VMMS). The service redirects writes to either the 3DFS datastore or the destination datastore.

10. After relocation completes, the 3DFS share is removed, and the UNC path is unregistered from CBFS.

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