To have RAM-based filesystem in OSX? I would like do real-time monitoring, discussing one part of it here about counting quickly. 
No packages about it in homebrew. 

How can you have RAM-based filesystem in OSX?
Mac OS includes support for RAM-based filesystems via ramfs. Everything is RAM backed so it's incredibly fast with near zero latency, but the contents are lost upon reboot.

To create one and mount it under /tmp/mymount, create a script with the following contents and run it:

#!/bin/sh
NUMSECTORS=128000       # a sector is 512 bytes
mydev=`hdiutil attach -nomount ram://$NUMSECTORS`
newfs_hfs $mydev
mkdir /tmp/mymount
mount -t hfs $mydev /tmp/mymount


The above was adapted from the hdiutil manual page

You can also do it in a one-liner (adapted from here):

diskutil erasevolume HFS+ 'RAM Disk' $(hdiutil attach -nomount ram://XXXXX)


Where XXXXX is the size in 2048 byte blocks. There is also a useful script in this post on Superuser.