Hello. The  Samsung 850 Pro SSDs are not validated or certified to work with Dell controllers and as such there is a communication mismatch between the drives and the controller at the firmware level. As a result, you are bound to realize unexpected poor Read and Write performance regardless of controller cache settings. Consider using Dell certified drives. Thank you.
I've seen several posts regarding this, but haven't seen a solution. I have some R710 servers with H700 controllers. I've installed Samsung 1TB 850 Pro SSDs and seeing very slow write performance compared to other SSDs tested within a same box, just slow in general. Samsung SSDs are doing ~500 MB/s in both directions when connected to standalone desktop, but ~500 MB/s read and ~45 MB/s writes when connected to a server. 1. 1x SSD in RAID 0 - No redundancy, just to test performance. I've tried every combination of write-back cache setting and read ahead setting possible (enabled, disabled) and never got more than ~45 MB/s writes. 2. 6x SSDs in RAID 5 with all possible combinations of write-back cache never getting more than ~110 MB/s writes. 3. 6x SSDs in RAID 10 again all possible settings of write-back cache - ~140MB/s. Lastly I've installed 120 GB Intel SSDs and got over 400 MB/s in both directions regardless of the RAID or caching. What is the problem with H700 controllers and Samsung SSDs? Does anyone know how to fix it? Any other R710 compatible controller that don't have this problem? JBOD capable, maybe? I've also tried an off the shelf PCIe controller without any luck (backplane compatibility issues or some sort) Thanks in advance. P.S. Controller is on a latest firmware along with BIOS and all the other components. Used Dell repository manager iDRAC discovery and bootable ISO to update everything. Samsung SSDs are on a latest firmware as well. 
Slow writes PERC H700 and Samsung 850 PRO SSD