The best answer is, it depends. And I know that sounds like a copout, but it really does.
If you perform Active Full Backups then stick with deduplication as that will catch duplicate blocks across all files on the volume. Remember, Veeam dedupe only occurs within a single backup file, such as data within a single VBK and not across multiple backup files for the same job. ReFS won't give much, if any, benefit when performing Active Fulls.
However, if you are performing Synthetic Fulls, then ReFS is a good candidate as it provides much faster creation of the synthetic full VBK file. Be sure to check the Veeam B&R console logs to confirm the job is using Fast Clone to perform the synthetic operation. This indicates that Veeam detected the backup repository is ReFS and is leveraging it for your synthetic full creation, which dramatically reduces the time to create the new VBK.
If you saw this week's Veeam Digest you will see that Microsoft have approved ReFS for use on SAN volumes (iSCSi/FC/FCoE) as long as the devices in the path, including the storage array are in the Windows Server Catalog (https://www.windowsservercatalog.com/
). Previously Microsoft would only support ReFS when using on direct-attached storage. To quote Gostev in the email, "ReFS is in fact fully supported on ANY storage hardware that is listed on Microsoft HCL". You can read about ReFS here: https://docs.microsoft.com/en-us/window ... s-overview
I work for NetApp so I'm happy to see that all of our current systems are fully supported by Microsoft with Windows Server 2016 so we can use ReFS for Backup Repositories and then use ONTAP's dedupe to get the best of both worlds
... although if you were using ONTAP you would be best to leverage ONTAP snapshots and SnapMirror for complete protection whilst still retaining Veeam's features like application-aware processing and SureBackups. But it's great to have the options now.