I don't think having a a 224MB /dev/shm uses 224MB of RAM, this is linux, not Windoze!
I have the following three tmpfs mounts:
Filesystem Size Used Avail Use% Mounted on
tmpfs 1001M 328K 1001M 1% /dev/shm
/tmp 1001M 9.3M 992M 1% /tmp
vartmp 1001M 165M 836M 17% /var/tmp
If you add the sizes up, you get 3,003MB (almost 3GB) on a laptop with only 2GB of RAM. System Monitor tells me that right now, I'm using 558MB (28%) of my RAM, and no swap. My guess is that tmpfs file systems use only as much RAM as they need. And if you get low on RAM, they'll get swapped out if need be. A good description of the usage of /dev/shm is availabe here
. My guess is, things could break badly if you umount /dev/shm.