SGDMA to SDRAM speed?

A descriptor is an instruction set that the master (in your case pc) gives the DMA that describes where to access data and how much data to take. In a memory-memory application like you have, the descriptor should contain the address locations of the source data (pcie) and the sink for the data (sdram) (or vice-versa) and the number of bytes to transfer in 1 pass or burst. With the mSGDMA you can write many descriptors into the descriptor buffer (see the dispatcher) and you can command the dispatcher to chain them: so once one descriptor is complete, the dma will automatically begin working on the next one. This way you can continuously transfer data.