what are _fsm and _vm files in postgres?

_fsm and _vm files will be not calculated while checking the table size.

_fsm :
                free space map file :how face free space pages available for usage.

_vm :   
               visibility map: what are the pages already occupied. excluding dead tuples.


e.g.,

$ cd <pg_base_dir/database_id>
-rw-------. 1 postgres postgres   8192 Apr 19 18:59 2608_vm
-rw-------. 1 postgres postgres  24576 Apr 19 18:59 2608_fsm
-rw-------. 1 postgres postgres 442368 Apr 19 18:59 2608
-rw-------. 1 postgres postgres   8192 Apr 19 18:59 1259_vm
-rw-------. 1 postgres postgres   8192 Apr 19 18:59 2619_vm
-rw-------. 1 postgres postgres  24576 Apr 19 18:59 2619_fsm

......