@gary GFS refers to long term retention on disk with Backup Copy jobs by keeping weekly, monthly, yearly backups. Since each of these is actually a full backup the disk space requirements are quite large. https://helpcenter.veeam.com/backup/hyp ... y_gfs.html
I understand how ReFS helps with forward forever incremental. When the oldest incremental is merged into the full backup file, instead of copying the data from the vib file into the vbk file, you can just update the metadata in the vbk file to point to the same physical blocks as the vib file.
If you are saying this also applies when you have multiple synthetic fulls with GFS, then this new ReFS functionality is even better than I thought!
The only argument for dedupe would be if you could get additional space savings when deduplicating between multiple backup jobs (or if you have per-vm backup files). However if you group VMs that tend to have similar data into the same job, there would be very little commonality between jobs anyway. With all the performance issues, overhead, cost, etc. of having to use a dedupe appliance, I see very little reason to use them at all. It's also another reason to stay away from NAS devices. I've always thought a Windows server with a ton of cheap internal storage was the best choice for backup repositories and this just confirms that