I have 2U PowerEdge R510 with PERC H700 Integrated and 8 installed disks, can someone tell me how to find if my disks are using the expander in the backplane or there are all directly attached to the controller. I am experiencing strange 500~550Mbytes/s read+write limitation with my 8 SSD disks in raid5. I know the controller has 2x4 internal connectors, so 8 disks can be direct attached, each with 6G supported? I know the server has 12 harddisk bays and I can use them all.
If you have two SFF-8087 ports on your controller, each one of those ports is a 4-lane SAS port. Natively, that would only be able to address EIGHT disks. With the use of an expander on the backplane, you can accommodate a greater number of disks.
So if your backplane can fit TWELVE drives, there's an expander on it. Also, look at the server. See if there's only one cable connecting the controller to the backplane.
-- Edit --
The Dell R510 contains a dual-ported SAS expander on the drive backplane. You're running into oversubscription issues on the SAS link. In addition, most RAID controllers of that level have diminishing returns at SSD quantities above 6 devices.