UNIX as a SQL database
UNIX is full of tables. When we talk about “processes”, we’re really referring to “rows in a process table.” When we talk about “file descriptors”, we’re really referring to “rows in a per-process file descriptor table”. There are other tables, too: a global file table, an inode table, routing tables, a mount table, page tables, and other tables I don’t know about.
These “tables” are custom in-memory data structures, but can be understood relationally. Here’s a simplified description of them in SQL.
-- One row = one process CREATE TABLE process ( pid INT PRIMARY KEY, -- ... ); -- Links processes to files via descriptors. -- In reality, may be implemented as per-process tables (blocks of memory). CREATE TABLE descriptor ( pid INT FOREIGN KEY process (pid), file_descriptor INT, file_id INT FOREIGN KEY file (id) PRIMARY KEY (pid, file_descriptor) ); -- Global file table CREATE TABLE file ( id INT AUTOINCREMENT PRIMARY KEY, offset_bytes INT NOT NULL, -- ... inode_number NOT NULL FOREIGN KEY inode_cache (inode_number) ); -- A cache of inodes on disk CREATE TABLE inode_cache ( inode_number INT PRIMARY KEY, -- ... ); CREATE TABLE pages ( pid INT FOREIGN KEY process (pid), virtual_page BITSTRING, physical_frame BITSTRING FOREIGN KEY ..., PRIMARY KEY (pid, virtual_page) );