Hello all, I just joined today. I have owned NAS units before, so I am not a complete rookie. 
I just purchased a new DS 1821+ and would like suggestions on how to get the best performance out of it. This is what I have. I have six hard drives. Two are 12TB, and the other four are 10TB. I have two NVMe 1TB each for cache. I have two 1TB SSD, which I can place in drive bays 7 and 8 if needed. I am using an 8-port 2.5 network switch and waiting for the 10-gig NIC card to arrive. I will also be installing 32 gigs of memory. The following is how the unit will be used. I will use the Active backup for business to backup one MAC and three PCs. I will use Adobe Lightroom for Photography work. My Lightroom catalog will be connected to my MAC by SSD, and I will use Synology Photos for family access. Plex server for music and movies. When in use, I will run an Ubuntu VM and one Minecraft container for the kids. I forgot I stored my documents in a shared folder on the NAS, not the computer. My question is: Is the cache read-only or read-write? Hard drive, one pool, one Volume, or another setup.
I am going to comment only on the Lightroom use…
In your computer box, the CPU box, put the LR catalog… preferably on a fast SSD. You CAN also put it on an external HDD ( or SSD version) but I do feel that will not be as fast.
Put your images on the NAS.
You probably already know that LR will not use a networked location for the catalog.
The method described is the fastest way to get access to any specific image, other than more SSDs in the CPU case.
Best you sync the catalog to a NAS location, just in case somersetting goes south..
Note that there are several files created in what we all call a catalog….. you need them all…. and backups for them all too.