According to suggestions in this post I am trying to improve my system to run better with a Solid State Drive. But regarding to RAMdisks and /etc/fstab usage I have some understanding problems coming up.

So let's say I add the following lines to /etc/fstab

tmpfs   /tmp       tmpfs   defaults,noatime,nodiratime,mode=1777   0  0
tmpfs   /var/spool tmpfs   defaults,noatime,nodiratime,mode=1777   0  0
tmpfs   /var/tmp   tmpfs   defaults,noatime,nodiratime,mode=1777   0  0
tmpfs   /var/log   tmpfs   defaults,noatime,nodiratime,mode=0755   0  0

I know that on startup these locations should now get mounted into the RAM (hopefully).

But what happens to the physical space that was mounted on those places before? Is it gone? Will it be back when I edit my /etc/fstab back to the Version without tmpfs? Will the space still be allocated on my SSD in a way that I can't use it for any other data?

Sometimes it is suggested to add the following line, too:

none /var/cache aufs dirs=/tmp:/var/cache=ro 0 0

What does this actually do? I noticed that /var/cache takes almost 1GB of space on my harddisk. So should i clear the directory before activating this line? (this is related to the former question)

This causes me some confusions and I hope you can give me some clarifications.

UPDATE
I've downloaded a image with 600MB in size into /tmp that is mounted with the tmpfs settings above. Now I wanted to compare the RAM usage before and after the download. I expect the RAM usage to be increased by 600MB after the download. But the System Monitoring Tool showed me no changes at all.

How can this be? Does tmpfs work other than I actually expect it to?