hi, firends, what's the difference between using PostgreSQL's share buffer and os's file buffer?

[sunpeng <bluevaley@xxxxxxxxx>]

hi, firends, 
    I have a 64bit 128GB machine, I have two choices:
           1.  I could set PostgreSQL share_buffer to a large value, such as 100GB, let os uses the remaining 28G memory for file system buffer
           2. I also could set PostgreSQL share_buffer to a small value, such as 10GB, let os uses the remaining 118G memory for file system buffer
    what's the difference between this two different methods in PostgreSQL?  Which one is prefered?
    Thanks!
peng