Finally i find solution that i never see on the web and this forum: 1- Power up your XPS 2- Use F12 to select "Device configuration" 3- when to go to "Device configuration" you can see Intel Rapid storage version on the top. i see Intel Rapid Storage 14.7 4-Exit and power up laptop and go to Windows 5-i uninstall 16 version and download/install Intel Rapid Storage 14 version 6-now acceleration option active and i used my 32GB SSD as Cache  So correct intel rapid storage selection is important to use SSD as Cache (i dont know why Dell added 16 version for XPS 9550 on download drivers!!!)
i buy Dell XPS 9550 second hand it show 2 storage on this PC: 1- 32 GB SSD (Windows installed on this drive) 2- 1 TB HDD i know 32 GB SSD used for HDD cache, but older user installed windows on this drive! so how i can use 32 GB SSD as HDD Cache? Please guide me
Dell XPS 9550 - How to use 32 GB SSD as HDD Cache