Best way how to write FPGA data in rt cRIO system in tdms file

Question

Hej,
I am struggling to write measured data from an analog input (NI 9215) sampled at up to 20 kHz to a tdms file in the rt system (crio-9022). I just need to save several periods of 4 arbitrary analog signals at frequencies between 5 Hz and 1KHz. So storing up to 50k values should already be enough.
I use a high priority and a low priority loop. First I tried to adapt the example from the "Getting Started with CompactRIO - Logging Data to Disk" (http://zone.ni.com/devzone/cda/tut/p/id/11198). But when I used this in my high priority loop (running at 1ms), the loop runs out of time and the rt system becomes unresponsible. If I change the number of elements to write (the number of elements to wait for in the fifo read block) it becomes better, but still data is lost because the loop finishes late.
So I was thinking to create a RT fifo and to store all the values from the measurement first in this memory inside the high priority loop and then write the values to the tdms file in the low priority loop. This time I used the read/write fpga block instead of the FPGA fifo block. It was already working better but writing the files to the tdms file took a lot of time since each value was read and written to the tdms file individually. Unfortunately I could not find a possibility how to write the whole rt fifo to the tdms file at once. Is there a block available or is it possible to create a big array first and then write the data to the tdms file at once? My code I tried is in this second picture.
I hope someone can give me some tips which method should be better for my project and a hint what I did wrong or what I can optimize. I stucked for days now on how to save my measurements on the cRIO system.
Thank you very much in advance. Have a nice weekend.
Best regards
Andy
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Answer 1

HiXiebo and Christian,
thank you very much for your answers. Actually, my high priority loop is much slower. I run it with a maximum loop time of 50us = 20kHz or slower, depending on my Signal I want to measure. So my data producing rate is maximum 4*8*20k=640 KB/s. My low priority loop runs at 2ms to 5 ms (much slower then the high priority loop), since I am doing just some simple math calculation there and control the front panel in this loop.
I understand that it is much more efficient to write blocks of data (e.g. 1024*32KB instead of just 32KB) to a file with TDMS. But is it also the same for a queue or RT FIFO, i.e. does the block size of the data chunks also matter for the queue and RT FIFO?
@Xiebo: I understand that caching the read data from the FPGA in the high priority loop first will improve my code. But I do not know how I can cache the data I read? I was thinking to do it with the FPGA FIFO, but the FPGA read/write blocks seem to be faster for me and I do not know why? Can you tell me a block/vi to cache the data I read from the FPGA or maybe even an example?
@Christian: This NI_MinimumBufferSize property looks exactly what I was looking for. But my question is now if I should put the tdms write VI's in my high priority loop and read directly from the FPGA FIFO buffer to the file as it is done in the Disk logging example at http://zone.ni.com/devzone/cda/tut/p/id/11198? Or is it better to read the data from the FPGA via the FPGA read/write function, write the data to a RT FIFO in the high priority loop and then write the data with the  NI_MinimumBufferSize property option to the tdms file from the RT FIFO in the low priority loop?
In summary, I am still unsure if the FPGA FIFO or the FPGA read/write function with a queue or RT FIFO is better for me and how I can create a cache to build chunks of data blocks to write.
Thank you very much in advance for your help.
Best regards
Andy