Efficient method to save data to disk in RT?

Question

What options are available to read data from a buffer and save it to the hard disk drive in RT? What method requires the least processor overhead...or perhaps can be set to run in the background without any processor intervention at all?

Answer 1

In Traditional NI-DAQ 7.0, buffered analog input (a la AI Read.vi) uses DMA to transfer points from an E-Series board to a software buffer on the PC. The uP doesn't have to intervene for this to occur thanks to DMA. However, the uP does have to read from the software buffer and write the points to disk.
In a real-time scenario, we generally recommend that customers acquire the data in their time-critical VI and then pass the data to a normal priority VI using RT FIFOs (shipping in 7.0, otherwise available on the web). The RT FIFOs are non-blocking queues that are expressly designed for moving data from a time-critical VI to a lower priority VI safely.
Acquired data would thus follow a path like this:
E-Series -> PC software buffer ->
time-critical VI in LabVIEW -> normal priority VI in LabVIEW -> disk
The first transfer from board to software buffer uses DMA and happens transparently to the user. The second transfer occurs within AI Read or AI Singlescan. Third transfer you must program using RT FIFOs. Last transfer you must program with File I/O VIs.