I use the following command to create a ram disk on boot:
/usr/sbin/diskutil erasevolume HFS+ "web-ram" `/usr/bin/hdiutil attach -notremovable -nomount ram://2200000`
This makes a ram disk thats about 1 GB. This ram disk holds nothing but tiny web files though. So in this case, it makes much more sense to use a much smaller block size. I'm thinking 0.5KB instead of the 4KB default of HFS+. I looked over the man page of diskutil, but it still is not clear at all how I can make the volume with a different block size (if at all?)