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 */)
