Accelerator for virtual machine backups

In the case of virtual machine backups (VMware, Hyper-V, or RHV), change tracking is usually done using special APIs provided by hypervisors or storage stack, hence track logs are not used or maintained for such backups. Instead of a track log, an extent file is maintained which is much smaller in size compared to a files/folder track log. The extent file is maintained along with the rest of the state files used in case of these backups. This extent file is used to identify where disk extends for a VMware backup are mapped into the previous image, this information is crucial to identify how to reconstruct a new image combining changed data and unchanged data from the previous image.

The extent file has no file-level information, only extent (data offset and size) information.

VMware Accelerator supports Full and Incremental images. Full VMware backup generated with Accelerator has the same layout as Non-accelerator backups. Incremental backup image is different, it reads only changed blocks and only those are sent from the backup host to the media server, but a full image is synthesized on the server. This allows such incremental images to be used for operations such as Instant Recovery / Instant Access. It also simplifies DR. On the flip side, as the size of the resultant image comparable to a full, which can have an adverse impact on tape-out performance. Replication performance for such images can also be impacted due to increased size, but MSDP and many other OST vendors take advantage of optimized duplication and impact is minimal in a homogeneous environment. There is no impact on the NetBackup catalog, the incremental image will catalog only those files that have changed. In other words, the catalog is the only key difference between Full and Incremental VMware backup images.