Buffered TDMS file still very large

Question

Hello, I am trying to to stream to a TDMS file. I am writing approximately 10 channels of data at 20 Hz. I was creating a new file evey 5 mins or so and defragging the file once I was finished with it, which was giving me a reasonable size of file. I decided that I would like to make bigger files (over the period of hours) and the the defragging would be too computationally costly to do on the fly. I have been trying to set a buffer for the TDMS files but the resultant files are very large. You can tell the file isn't being writen until its closed (from watching its size in windows explorer) which suggests the file is being buffered. However its about 6 times bigger than an ASCII file of the same data, with and index almost as big again.
Does anyone have any ideas why it would appear to buffer but not actually  reduce the data size. I'm running Windows XP, LabVIEW 10.0.
When I create each new file I run this to set the buffers. (I have also hard wired in a buffersize of 100000 on a one minute file to no avail.
Niall
Thanks
Niall

Answer 1

I has this marked as the solution but it turns out it only helped one of my programs. I am still getting very large (fragmented) TDMS and index files for another program. The problem is that if I just defrag the (supposedly buffered) file, it interupts the data logging because it takes so long. I'm 99% sure that it *is* buffereing as it doesn't write till it closes the file, and if I use the read properties function it reads back a set buffer size. Here is the VI that actually writes the data. There bit at the top is for writing an optional acii file so you can ignore that.
Its maybe a bit hard to see whats going on in the next one, but this is where the file is created before being passed to the setbuffer VI which I posted earlier. It also closes that last file
Its really hacking me off now and holding me up from going no to other stuff. It would be great if someone had some ideas.
Thanks
Niall