About the names:
Back in PureDisk's time, Symantec used to call it "Virtual Synthetic Backups".
Now all documents refer to it as "Optimized Synthetic Backups".
In NetBackup, "Optimized Synthetic Backups" and "Synthetic Backups" achieve the same thing, essentially.
The core difference lies in how backups are synthesized.
In Synthetic Backup:
- Full-image traverses from the storage (disk or tape) to the media server via whatever connection. (Fiber, Ethernet, SAS, iScsi, etc...)
- Incremental-image(s) also traverses from the storage to the media server.
- Media server uses its resources (processor, memory, network, storage i/o, etc) to combine the two to create a new "full" backup. I.e., It synthesizes a new full-image out of a previous full-image and its incremental-image(s).
- Media server then sends this new synthesized image back to the storage via whatever connection.
In Optimized Synthetic Backup:
- Media server 'instructs' the storage server (via the OST API) to synthesize a new image.
- Without sending anything to the media server, the storage server synthesizes the new image out of existing images. This is all done inside the storage itself. The media server resources are not used.
- When the new synthetic image is successfully created, the storage server 'informs' (via the OST API) the media server mission accomplished.
Compare the two outcomes, and you will see that the result is exactly the same.
It is just that one does it locally inside the storage, without bothering a media server.
Sometimes people get confused because when you are using MSDP, the storage server and a media server share the same host. This makes the whole concept of "synthesize locally inside the storage without bothering a media server" a little vague.
But then, if you have multiple media servers with only one storage server on one of them, then this relationship can be observed a bit more clearly. Of couse there are also the dedicated storage server appliances used for OST based deduplication storage.
Note that some other vendors refer to "Virtual Synthetic Backups" and "Synthetic Backups" interchangeably. They add the word "Virtual" whenever they feel like being redundant.
Either way, they are, in most cases, not talking about doing it locally inside the storage as in Netbackup's "Optimized Synthetic Backup".