Buffer cache mechanism

Hi All, I was reading through the text, "The Design of the Unix Operating System" - Maurice Bach While reading through the Buffer cache, I had the following query Buffer data structure, has Buffer header + pointer to data area + pointers for Buffer Management. Does this,"pointer to data area", corresponds to disk block or Logical Filesystem block ? I am confused about this. To add more, I am aware of the fact that disk block is of 512 bytes on the most of the hard disks and logical disk block size ( depending on filesystem ) is of 4k in size. what would be available in data area pointed in buffer ? Thanks, Satheesaran