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 */)