linux memory usage

["Taylor Lewick" <tlewick (at) tradebotsystems.com>]

When I pull up hobbit graphs for memory for one of our database servers with 16 GB of memory, I see the Actual Memory Used is reporting as 657 M and swap is 0 M Used.

But if I run ps -vax --sort -rss  note the top 4 processes...
Looks like just over 7.0 GB of memory is consumed.

I'm deleting PID, TTY and STAT colum to make sure it all fits on one line.

TIME  MAJFL   TRS   DRS   RSS %MEM COMMAND
12:39      4  3519 3310360 3193428 19.4 postgres: 
 4:22      0  3519 3312708 3077228 18.7 postgres: 
 0:31      0  3519 3312832 993816  6.0 postgres: 
 0:26      0  3519 3312128 192740  1.1 postgres: 

Output of free shows...
		total       used       free     shared    buffers     cached
Mem:      16433348   16252424     180924         0     168500   15409800
-/+ buffers/cache:     674124   15759224
Swap:      8355832        540    8355292

So is the system telling me that most of the memory the processes are taking up from the ps -vax output is cached and can be released at any time?

Thanks,