How the NetBackup Accelerator works with virtual machines
For Azure and Azure Stack Hub backups, Accelerator is activated when you select an Accelerator supported storage type, like MSDP, OpenStorage, CloudStorage, and MSDP-C (Azure and AWS).
The NetBackup Accelerator creates the backup stream and backup image for each virtual machine as follows:
If the virtual machine has no previous backup, NetBackup performs a full backup.
At the next backup, NetBackup identifies data that has changed since the previous backup. Only changed blocks and the header information are included in the backup, to create a full VM backup. The changed blocks are identified by comparing the previous reference snapshot and the current snapshot. If you select the Keep backup only or Initiate backup when snapshot is about to expire option in the protection plan, the snapshot is retained for accelerator purposes till the next backup is completed.
The backup host sends to the media server a tar backup stream that consists of the following: The virtual machine's changed blocks, and the previous backup ID and data extents (block offset and size) of the unchanged blocks.
The media server reads the virtual machine's changed blocks, the backup ID, and information about the data extents of the unchanged blocks. From the backup ID and data extents, the media server locates the rest of the virtual machine's data in existing backups.
The media server directs the storage server to create a new full image that consists of the following: The newly changed blocks, and the existing unchanged blocks that reside on the storage server. The storage server may not write the existing blocks but rather link them to the image.
Microsoft Azure does not allow more than 200 subsequent incremental snapshots. If you select the option in the protection plan and specify such a retention period for the snapshot, it leads to more than 200 incremental snapshots. Then, full backups take place instead of the accelerator. It is recommended to keep a reasonable snapshot retention period to utilize the accelerator benefits.
If the configuration of a VM changes, for example, if a new disk is added to a VM between two accelerator backups, a full backup is taken for that disk, and accelerator backup is taken for the existing disks.