$PostgreSQL: pgsql/src/backend/storage/smgr/README,v 1.6 2008/08/11 11:05:11 heikki Exp $

Storage Manager
===============

In the original Berkeley Postgres system, there were several storage managers,
of which only the "magnetic disk" manager remains.  (At Berkeley there were
also managers for the Sony WORM optical disk jukebox and persistent main
memory, but these were never supported in any externally released Postgres,
nor in any version of PostgreSQL.)  However, we retain the notion of a storage
manager switch in case anyone wants to reintroduce other kinds of storage
managers.

In Berkeley Postgres each relation was tagged with the ID of the storage
manager to use for it.  This is gone.  It would be more reasonable to
associate storage managers with tablespaces (a feature not present as this
text is being written, but one likely to emerge soon).

The files in this directory, and their contents, are

    smgrtype.c	Storage manager type -- maps string names to storage manager
		IDs and provides simple comparison operators.  This is the
		regproc support for type 'smgr' in the system catalogs.
		(This is vestigial since no columns of type smgr exist
		in the catalogs anymore.)

    smgr.c	The storage manager switch dispatch code.  The routines in
		this file call the appropriate storage manager to do hardware
		accesses requested by the backend.  smgr.c also manages the
		file handle cache (SMgrRelation table).

    md.c	The magnetic disk storage manager.

Note that md.c in turn relies on src/backend/storage/file/fd.c.

Relation Forks
==============

Since 8.4, a single smgr relation can be comprised of multiple physical
files, called relation forks. This allows storing additional metadata like
Free Space information in additional forks, which can be grown and truncated
independently of the main data file, while still treating it all as a single
physical relation in system catalogs.

It is assumed that the main fork, fork number 0 or MAIN_FORKNUM, always
exists. Fork numbers are assigned in src/include/storage/relfilenode.h.
Functions in smgr.c and md.c take an extra fork number argument, in addition
to relfilenode and block number, to identify which relation fork you want to
access. Since most code wants to access the main fork, a shortcut version of
ReadBuffer that accesses MAIN_FORKNUM is provided in the buffer manager for
convenience.
