海山数据库(He3DB)源码详解：海山MySQL redo日志-写入过程

# 一、redo log block

设计InnoDB时为了更好的进行系统奔溃恢复，将通过mtr生成的redo日志放在大小为512字节的页中。为了和表空间中的页做区别，于是把用来存储redo日志的页称为block。一个redo log block的示意图如下：

真正的redo日志都是存储到占用496字节大小的log block body中。另外，log block header占12个字节，log block trailer占4个字节，存储的是一些管理信息。

其中，log block header中包含以下属性字段：

• LOG_BLOCK_HDR_NO(4B)：每一个block都有一个大于0的唯一标号，该属性就表示该标号值。
• LOG_BLOCK_HDR_DATA_LEN(2B)：表示block中已经使用了多少字节，初始值为12。随着往block中写入的redo日志越来也多，该值也跟着增长。如果log block body已经被全部写满，那么值被设置为512。
• LOG_BLOCK_FIRST_REC_GROUP(2B)：一条redo日志也可以称之为一条redo日志记录，一个mtr会生产多条redo日志记录，这些redo日志记录被称之为一个redo日志记录组（redo log record group）。LOG_BLOCK_FIRST_REC_GROUP就代表该block中第一个mtr生成的redo日志记录组的偏移量，即这个block里第一个mtr生成的第一条redo日志的偏移量。
• LOG_BLOCK_CHECKPOINT_NO(4B)：表示checkpoint的序号。

log block trailer中包含的属性字段为：

• LOG_BLOCK_CHECKSUM(4B)：表示block的校验值，用于正确性校验。

二、redo日志缓冲区

与为了解决磁盘速度过慢的问题而引入了Buffer Pool的思想类似，写入redo日志时也不能直接直接写到磁盘上。实际上在服务器启动时就向操作系统申请了一大⽚称之为redo log buffer的连续内存空间，即redo日志缓冲区，也可以简称为log buffer。这⽚内存空间被划分成若干个连续的redo log block，如图所示：

三、redo日志写入log buffer

向log buffer中写入redo日志的过程是顺序的，也就是先往前边的block中写，当该block的空闲空间用完之后再往下一个block中写。因此，当往log buffer中写入redo日志时，第一个遇到的问题就是应该写在哪个block的哪个偏移量处，所以InnoDB的特意提供了一个称之为buf_free的全局变量，该变量指明后续写入的redo日志应该写入到log buffer中的哪个位置。

由于一个mtr执行过程中可能产生若干条redo日志，这些redo日志是一个不可分割的组，所以其实并不是每生成一条redo日志，就将其插入到log buffer中，而是每个mtr运行过程中产生的日志先暂时存到一个地方，当该mtr结束的时候，将过程中产生的一组redo日志再全部复制到log buffer中。

四、源码解析

4.1 log buffer结构体

/** Redo log buffer */
struct log_t{
   
   
	char		pad1[CACHE_LINE_SIZE];
					/*!< Padding to prevent other memory
					update hotspots from residing on the
					same memory cache line */
	lsn_t		lsn;		/*!< log sequence number */
	ulint		buf_free;	/*!< first free offset within the log
					buffer in use */
	byte*		buf_ptr;	/*!< unaligned log buffer, which should
					be of double of buf_size */
	byte*		buf;		/*!< log buffer currently in use;
					this could point to either the first
					half of the aligned(buf_ptr) or the
					second half in turns, so that log
					write/flush to disk don't block
					concurrent mtrs which will write
					log to this buffer */
	bool		first_in_use;	/*!< true if buf points to the first
					half of the aligned(buf_ptr), false
					if the second half */
	ulint		buf_size;	/*!< log buffer size of each in bytes */
	ulint		max_buf_free;	/*!< recommended maximum value of
					buf_free for the buffer in use, after
					which the buffer is flushed */
	bool		check_flush_or_checkpoint;
					/*!< this is set when there may
					be need to flush the log buffer, or
					preflush buffer pool pages, or make
					a checkpoint; this MUST be TRUE when
					lsn - last_checkpoint_lsn >
					max_checkpoint_age; this flag is
					peeked at by log_free_check(), which
					does not reserve the log mutex */
	UT_LIST_BASE_NODE_T(log_group_t)
			log_groups;	/*!< log groups */

#ifndef UNIV_HOTBACKUP
	/** The fields involved in the log buffer flush @{ */

	ulint		buf_next_to_write;/*!< first offset in the log buffer
					where the byte content may not exist
					written to file, e.g., the start
					offset of a log record catenated
					later; this is advanced when a flush
					operation is completed to all the log
					groups */
	volatile bool	is_extending;	/*!< this is set to true during extend
					the log buffer size */
	lsn_t		write_lsn;	/*!< last written lsn */
	lsn_t		current_flush_lsn;/*!< end lsn for the current running
					write + flush operation */
	lsn_t		flushed_to_disk_lsn;
					/*!< how far we have written the log
					AND flushed to disk */
	ulint		n_pending_flushes;/*!< number of currently
					pending flushes; incrementing is
					protected by the log mutex;
					may be decremented between
					resetting and setting flush_event */
	os_event_t	flush_event;	/*!< this event is in the reset state
					when a flush is running; a thread
					should wait for this without
					owning the log mutex, but NOTE that
					to set this event, the
					thread MUST own the log mutex! */
	ulint		n_log_ios;	/*!< number of log i/os initiated thus
					far */
	ulint		n_log_ios_old;	/*!< number of log i/o's at the
					previous printout */
	time_t		last_printout_time;/*!< when log_print was last time
					called */
	/* @} */

	/** Fields involved in checkpoints @{ */
	lsn_t		log_group_capacity; /*!< capacity of the log group; if
					the checkpoint age exceeds this, it is
					a serious error because it is possible
					we will then overwrite log and spoil
					crash recovery */
	lsn_t		max_modified_age_async;
					/*!< when this recommended
					value for lsn -
					buf_pool_get_oldest_modification()
					is exceeded, we start an
					asynchronous preflush of pool pages */
	lsn_t		max_modified_age_sync;
					/*!< when this recommended
					value for lsn -
					buf_pool_get_oldest_modification()
					is exceeded, we start a
					synchronous preflush of pool pages */
	lsn_t		max_checkpoint_age_async;
					/*!< when this checkpoint age
					is exceeded we start an
					asynchronous writing of a new
					checkpoint */
	lsn_t		max_checkpoint_age;
					/*!< this is the maximum allowed value
					for lsn - last_checkpoint_lsn when a
					new query step is started */
	ib_uint64_t	next_checkpoint_no;
					/*!< next checkpoint number */
	lsn_t		last_checkpoint_lsn;
					/*!< latest checkpoint lsn */