I had the same issue and here is the solution that worked with me.
So I had worked with a guy from EMC support named Joseph. He is LV3 united states Sev 1 support. This guy is the big guns they call out when everyone is stuck (Getting ready to make a big purchase with EMC always helps when needing support
). Anyway I was also having a replication issues after going to Veeam 9. Not sure what changed in Veeam and causes it but we figured out a solution with the help of Joe in EMC support and fixed it.
We found that there are only so many streams on a DD. Which we knew but we did not know is that there is a limit to how many streams an Mtree can use which I couldn't believe no one in lower support knew. You can have 196 streams like our new DD4200 but each Mtree has a limit on how much it can use for replication. So when you hit the limit it is capped and will not go any faster so you never fully saturate your link or push it to what it can do.
We had one giant Mtree for Veeam as we have other backup products. What Joe with EMC support did was create 6 more Mtrees. He then setup DDBoost for these Mtrees and we worked to spread out our jobs between them. We disabled the job and used fast copy on the DD to move the data over to the new Mtree. Once that was done we created new repositories for the data in the new Mtree and then in the job, we changed the path to point to the new repository. We then enabled the job and ran it and ensure all is ok. We then set up new replication points for the new Mtrees.
This kinda freaks Veeam out a little and puts the old backup directory and job in Disk (imported) but that is ok because when you use fast copy it will be a mirror copy of the data so it thinks that is old data. If you look at the job though it has all the data in the new location. I gave it a good week and then deleted the old job from Disk(Imported). Joe said it was always wise to just give it a few days. Then I went thru and deleted the old repository if it was not being used by anything else.
By doing this my replication has not had an issue.