Thanks for your response.

I mean ,I still have the same probleme since the day I opened this thread ,and your response on the post #4 give me some hope already 3 years ago about the possibility to using sparsefiles on my Synology NAS.
I use Jdownloader everyday for download files on my NAS but I can't able to use my full download speed because I can't use sparsefiles (with only 1 connection per filehosts, the speed download can't reach even 50% of my max speed ,contrary when I download on my USB3 HDD I can reach the max speed ,when using sparsefiles).

I hope to find a solution one day to this problem because I love to use jdownloader too much for quit him.