Free Space Map ( FSM) Vs Visibility Map (VM)
Free Space Map ( FSM)
When inserting a heap or an index tuple, PostgreSQL uses the FSM of the corresponding table or index to select the page which can be inserted it. All tables and indexes have respective FSMs. Each FSM stores the information about the free space capacity of each page within the corresponding table or index file. All FSMs are stored with the suffix 'fsm', and they are loaded into shared memory if necessary. The extension pg_freespacemap provides the freespace of the specified table/index.
Visibility Map ( VM)
Vacuum processing is costly; thus, the VM was been introduced in version 8.4 to reduce this cost. The basic concept of the VM is simple. Each table has an individual visibility map that holds the visibility of each page in the table file. The visibility of pages determines whether each page has dead tuples. Vacuum processing can skip a page that does not have dead tuples.
Suppose that the table consists of three pages, and the 0th and 2nd pages contain dead tuples and the 1st page does not. The VM of this table holds information about which pages contain dead tuples. In this case, vacuum processing skips the 1st page by referring to the VM's information.
