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)
);
Tagged . All content copyright James Fisher 2017. This post is not associated with my employer.
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