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First, I'm a big Synology fan for over 10 years now and I have had many Synology products in my setup, including 6 NAS's now. So, Synology DSM is always easy to use and this NAS is no exception. It did take a while to get the RAID 5 array consistency check done (over 1 day), but once up and running, no issues. My beef with this comes with RSYNC speed and the price you are paying for this NAS. It has a slower boot up time than its predecessors and slower RSYNC times than its predecessors. This is a 64 bit processor (which is the main reason I bought it) and it does allow you to build a single volume over 16TB, unlike the 32 bit variants of their rackmount units before it, which only allow up to a 16TB single volume. However, this is a Realtek processor. As you take a look through the Synology lineup, you will see many different processors, Like Intel Celeron, Realtek, Intel Atom, and Intel Xeon.
I have an RS816 that actually has a higher transfer rate on RSYNC by over 10mbps. The RS816 also boots ~7 seconds faster. The Synology DS918+ by comparison (a desktop NAS, not rackmount) boots significantly faster and has over 20mbps higher RSYNC backup speed.
For Windows transfer to the NAS via SMB or Acronis backup, I was getting roughly the same backup speed for a 130GB image as I was on the other NAS's in the rack. But, for my use case, which is backing up all other NAS's in the rack to the RS819 for a second copy (I'm a photographer, so I can't lose photos), this is a bit slow and their DS918+ desktop NAS is much faster at ~$50.00 less.
The Synology rack series is built to a bit higher standard than the desktop NAS’s and I believe the rack series are really targeted at more of the business side of things. At the end of the day, you have to consider the use cases and see if this is the right NAS for those use cases. Again, in my particular instance, most of the RSYNC work is done at night while I’m sleeping, but the initial RSYNC seeding of the data was slower on this RS819 by a significant amount, than it was on other NAS’s in my rack. If used for mostly Windows transfer, this unit works great for that.