Buffer Overview

核心价值

• 读优化: 内存比磁盘快几个数量级。数据页首次读取后缓存在内存，后续访问直接命中内存，不再触发慢速磁盘 I/O。
• 写优化: 修改数据时先只改内存（标记为脏页），然后通过后台进程异步、批量刷回磁盘。避免每次修改都直接卡在慢速磁盘写上。

缓存结构

BufferMapping

• SharedBufHash
• buf_table.c
• mapping BufferTags to buffer indexes

/* entry for buffer lookup hashtable */
typedef struct
{
	BufferTag	key;			/* Tag of a disk page */
	int			id;				/* Associated buffer ID */
} BufferLookupEnt;

typedef struct buftag
{
	Oid			spcOid;			/* tablespace oid */
	Oid			dbOid;			/* database oid */
	RelFileNumber relNumber;	/* relation file number */
	ForkNumber	forkNum;		/* fork number */
	BlockNumber blockNum;		/* blknum relative to begin of reln */
} BufferTag;

BufferDescriptors

• BufferDescPadded *BufferDescriptors;

typedef struct BufferDesc
{
	BufferTag	tag;			/* ID of page contained in buffer */
	int			buf_id;			/* buffer's index number (from 0) */

	/* state of the tag, containing flags, refcount and usagecount */
	pg_atomic_uint32 state;

	int			wait_backend_pgprocno;	/* backend of pin-count waiter */
	int			freeNext;		/* link in freelist chain */
	LWLock		content_lock;	/* to lock access to buffer contents */
} BufferDesc;

BufferBlocks

• char *BufferBlocks;
• shared memory
• shared_buffers = 128M

其他缓存

Ring Buffer

When reading or writing a huge table, PostgreSQL uses a ring buffer instead of the buffer pool.

The ring buffer is a small, temporary buffer area. It is allocated in shared memory when any of the following conditions is met:

1. Bulk-reading: When scanning a relation whose size exceeds one-quarter of the buffer pool size (shared_buffers/4). In this case, the ring buffer size is 256 KB.

2. Bulk-writing: When executing the following SQL commands, the ring buffer size is 16 MB:

   • COPY FROM command.
   • CREATE TABLE AS command.
   • CREATE MATERIALIZED VIEW or REFRESH MATERIALIZED VIEW command.
   • ALTER TABLE command.

3. Vacuum-processing: When an autovacuum process performs vacuuming. In this case, the ring buffer size is 256 KB.

Local Buffer

When a backend creates a temporary table, the buffer manager allocates a memory area for the backend and creates a local buffer.

脏页落盘

• Checkpointer: 缩短崩溃恢复（Crash Recovery）时间
• Background Writer: 保证 Backend 进程随时有干净的 Buffer 可用(提前把 nextVictimBuffer 指针前方的脏页刷出)

参考文档

• https://www.interdb.jp/pg/pgsql08/index.html