innodb redo buffer的认识

                           InnoDB Redo的结构漫谈（1）
                           
                           作者：@明天会更好(群号：196380905）
                           
1.Redo 日志：
        redo日志的结构是物理到页，逻辑到记录；采用的是物理逻辑日志。innodb redo日志是基于mini-transaction实现的。
        每个重做日志块是512个字节，也就意味着log buffer中也是以512个字节的块组织的。每个mini-transaction 会有一
        个mtr的结构，这个mtr的结构封装了未被写入到log buffer的日志，mtr结构有log成员，该成员就是日志的结构。log_t
        是代表log buffer的结构，mysqld运行实例中只维护一个sys_log实例(lot_t sys_log).
            sys_log维护了我们制定的innodb_log_buffer_size制定的大小的redo buffer.该内存区域在sys_log->buf维护了这个内存区域。
            
                
                
                redo buffer 有如下三个结构：
                
                |----------------|
                |  block header  |
                |----------------|
                |  records  logs |
                |----------------|
                |  block trialer |         
                |----------------|
                  (log block 结构)
                  
         |----------|----------|----------|----------|
         |log block |log block |log block |log block |
         |----------|----------|----------|----------|
                    (log buffer 组织方式)
                    
    log buffer 中的头和尾部在源码中定义：
      /* Offsets of a log block header */
        #define    LOG_BLOCK_HDR_NO    0    /* block number which must be > 0 and
                    is allowed to wrap around at 2G; the
                    highest bit is set to 1 if this is the
                    first log block in a log flush write
                    segment */
        #define LOG_BLOCK_FLUSH_BIT_MASK 0x80000000UL
                    /* mask used to get the highest bit in
                    the preceding field */
        #define    LOG_BLOCK_HDR_DATA_LEN    4    /* number of bytes of log written to
                    this block */
        #define    LOG_BLOCK_FIRST_REC_GROUP 6    /* offset of the first start of an
                    mtr log record group in this log block,
                    0 if none; if the value is the same
                    as LOG_BLOCK_HDR_DATA_LEN, it means
                    that the first rec group has not yet
                    been catenated to this log block, but
                    if it will, it will start at this
                    offset; an archive recovery can
                    start parsing the log records starting
                    from this offset in this log block,
                    if value not 0 */
        #define LOG_BLOCK_CHECKPOINT_NO    8    /* 4 lower bytes of the value of
                    log_sys->next_checkpoint_no when the
                    log block was last written to: if the
                    block has not yet been written full,
                    this value is only updated before a
                    log buffer flush */
        #define LOG_BLOCK_HDR_SIZE    12    /* size of the log block header in
                    bytes */

        /* Offsets of a log block trailer from the end of the block */
        #define    LOG_BLOCK_CHECKSUM    4    /* 4 byte checksum of the log block
                    contents; in InnoDB versions
                    < 3.23.52 this did not contain the
                    checksum but the same value as
                    .._HDR_NO */
        #define    LOG_BLOCK_TRL_SIZE    4    /* trailer size in bytes */
    
    mini-transactions是mtr的缩写，mtr会把一组log record集中起来，然后写入到log buffer.
        /* Mini-transaction handle and buffer */
            struct mtr_t{
            #ifdef UNIV_DEBUG
                ulint        state;    /*!< MTR_ACTIVE, MTR_COMMITTING, MTR_COMMITTED */
            #endif
                dyn_array_t    memo;    /*!< memo stack for locks etc. */
                dyn_array_t    log;    /*!< mini-transaction log */
                unsigned    inside_ibuf:1;
                            /*!< TRUE if inside ibuf changes */
                unsigned    modifications:1;
                            /*!< TRUE if the mini-transaction
                            modified buffer pool pages */
                unsigned    made_dirty:1;
                            /*!< TRUE if mtr has made at least
                            one buffer pool page dirty */
                ulint        n_log_recs;
                            /* count of how many page initial log records
                            have been written to the mtr log */
                ulint        n_freed_pages;
                            /* number of pages that have been freed in
                            this mini-transaction */
                ulint        log_mode; /* specifies which operations should be
                            logged; default value MTR_LOG_ALL */
                lsn_t        start_lsn;/* start lsn of the possible log entry for
                            this mtr */
                lsn_t        end_lsn;/* end lsn of the possible log entry for
                            this mtr */
            #ifdef UNIV_DEBUG
                ulint        magic_n;
            #endif /* UNIV_DEBUG */
            };

            dyn_block_t的定义：

            /** A block in a dynamically allocated array */
            struct dyn_block_t;
            /** Dynamically allocated array */
            typedef dyn_block_t        dyn_array_t;

            /** This is the initial 'payload' size of a dynamic array;
            this must be > MLOG_BUF_MARGIN + 30! */
            #define    DYN_ARRAY_DATA_SIZE    512
            /*#################################################################*/

            /** @brief A block in a dynamically allocated array.
            NOTE! Do not access the fields of the struct directly: the definition
            appears here only for the compiler to know its size! */
            struct dyn_block_t{
                mem_heap_t*    heap;    /*!< in the first block this is != NULL
                            if dynamic allocation has been needed */
                ulint        used;    /*!< number of data bytes used in this block;
                            DYN_BLOCK_FULL_FLAG is set when the block
                            becomes full */
                byte        data[DYN_ARRAY_DATA_SIZE]; //真实的日志数据
                            /*!< storage for array elements */
                UT_LIST_BASE_NODE_T(dyn_block_t) base;
                            /*!< linear list of dyn blocks: this node is
                            used only in the first block */
                UT_LIST_NODE_T(dyn_block_t) list;
                            /*!< linear list node: used in all blocks */
            #ifdef UNIV_DEBUG
                ulint        buf_end;/*!< only in the debug version: if dyn
                            array is opened, this is the buffer
                            end offset, else this is 0 */
                ulint        magic_n;/*!< magic number (DYN_BLOCK_MAGIC_N) */
            #endif
            };


2.mini-transaction log的调用方法的逻辑：

   mtr_commit
     ->mtr_log_reserve_and_write
        ->log_write_low //该函数是把mtr->log写入到空闲的log buffer中
        /************************************************************//**
        Writes the contents of a mini-transaction log, if any, to the database log. */
        static void mtr_log_reserve_and_write(mtr_t*    mtr)    /*!< in/out: mtr */
                --->
                /************************************************************//**
                Writes to the log the string given. It is assumed that the caller holds the
                log mutex. */
                UNIV_INTERN void log_write_low(byte*    str/*!< in: string */,        ulint    str_len/*!< in: string length */)