Jessie already has a number of RAM disks (use the df command and note the 'tmpfs' = RAM disk entries).
The good thing about tmpfs is that it 'auto allocates' the space up to the maximum allowed - so the fact that by Jessie already has 3 RAM disks of 200Mb each is irrelevant - that's the max. not the actual used As with everything *nix, creating a RAM disk is 'non-intuitive' but 'easy when you know how'. You start by defining a 'folder name' - for example 'ram-disk'. This is best placed at 'root' (/) so it's easy to find, however I placed mine within a folder I 'shared' with my PC :- sudo mkdir /my-folder-for-PC/ram-disk by default, it's going to be 'read only', so you will have to 'sudo' when you want to write it (and you won't be able to do that from the PC). Instead, chmod :- sudo chmod +777 /my-folder-for-PC/ram-disk Next you 'link' the just created folder to the RAM-disk driver tmpfs in the 'file system definitions table', fstab :- sudo nano /etc/fstab to define a RAM disk that can expand to 125Mb (10s test at 5fps = 50 jpeg photos allowing 2.5Mb ea.), add the line :- tmpfs /my-folder-for-PC/ram-disk tmpfs nodev,nosuid,size=125M 0 0 then 'output' (Ctrl o) the file and 'exit' (Ctrl x) the edit
To 'mount' the RAM-disk (without rebooting the Pi), type :-
sudo mount -a
To check all is OK, use the df command