Postgresql文件管理器一图流

	/*
	 * for md.c; per-fork arrays of the number of open segments
	 * (md_num_open_segs) and the segments themselves (md_seg_fds).
	 */
	int			md_num_open_segs[MAX_FORKNUM + 1];
	struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1];

/*
 * smgr.c maintains a table of SMgrRelation objects, which are essentially
 * cached file handles.  An SMgrRelation is created (if not already present)
 * by smgropen(), and destroyed by smgrclose().  Note that neither of these
 * operations imply I/O, they just create or destroy a hashtable entry.
 * (But smgrclose() may release associated resources, such as OS-level file
 * descriptors.)
 *
 * An SMgrRelation may have an "owner", which is just a pointer to it from
 * somewhere else; smgr.c will clear this pointer if the SMgrRelation is
 * closed.  We use this to avoid dangling pointers from relcache to smgr
 * without having to make the smgr explicitly aware of relcache.  There
 * can't be more than one "owner" pointer per SMgrRelation, but that's
 * all we need.
 *
 * SMgrRelations that do not have an "owner" are considered to be transient,
 * and are deleted at end of transaction.
 */
typedef struct SMgrRelationData
{
	/* rnode is the hashtable lookup key, so it must be first! */
	RelFileNodeBackend smgr_rnode;	/* relation physical identifier */

	/* pointer to owning pointer, or NULL if none */
	struct SMgrRelationData **smgr_owner;

	/*
	 * These next three fields are not actually used or manipulated by smgr,
	 * except that they are reset to InvalidBlockNumber upon a cache flush
	 * event (in particular, upon truncation of the relation).  Higher levels
	 * store cached state here so that it will be reset when truncation
	 * happens.  In all three cases, InvalidBlockNumber means "unknown".
	 */
	BlockNumber smgr_targblock; /* current insertion target block */
	BlockNumber smgr_fsm_nblocks;	/* last known size of fsm fork */
	BlockNumber smgr_vm_nblocks;	/* last known size of vm fork */

	/* additional public fields may someday exist here */

	/*
	 * Fields below here are intended to be private to smgr.c and its
	 * submodules.  Do not touch them from elsewhere.
	 */
	int			smgr_which;		/* storage manager selector */

	/*
	 * for md.c; per-fork arrays of the number of open segments
	 * (md_num_open_segs) and the segments themselves (md_seg_fds).
	 */
	int			md_num_open_segs[MAX_FORKNUM + 1];
	struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1];

	/* if unowned, list link in list of all unowned SMgrRelations */
	dlist_node	node;
} SMgrRelationData;

typedef SMgrRelationData *SMgrRelation