zz MySQL redo log及recover过程浅析
原作地址:http://www.cnblogs.com/liuhao/p/3714012.html
写在前面:作者水平有限,欢迎不吝赐教,一切以最新源码为准。
InnoDB redo log
669 /* Offsets of a log file header */
670 #define LOG_GROUP_ID 0 /* log group number */
671 #define LOG_FILE_START_LSN 4 /* lsn of the start of data in this
672 log file */
673 #define LOG_FILE_NO 12 /* 4-byte archived log file number;
674 this field is only defined in an
675 archived log file */
676 #define LOG_FILE_WAS_CREATED_BY_HOT_BACKUP 16
677 /* a 32-byte field which contains
678 the string 'ibbackup' and the
679 creation time if the log file was
680 created by ibbackup --restore;
681 when mysqld is first time started
682 on the restored database, it can
683 print helpful info for the user */
684 #define LOG_FILE_ARCH_COMPLETED OS_FILE_LOG_BLOCK_SIZE
685 /* this 4-byte field is TRUE when
686 the writing of an archived log file
687 has been completed; this field is
688 only defined in an archived log file */
689 #define LOG_FILE_END_LSN (OS_FILE_LOG_BLOCK_SIZE + 4)
690 /* lsn where the archived log file
691 at least extends: actually the
692 archived log file may extend to a
693 later lsn, as long as it is within the
694 same log block as this lsn; this field
695 is defined only when an archived log
696 file has been completely written */
697 #define LOG_CHECKPOINT_1 OS_FILE_LOG_BLOCK_SIZE
698 /* first checkpoint field in the log
699 header; we write alternately to the
700 checkpoint fields when we make new
701 checkpoints; this field is only defined
702 in the first log file of a log group */
703 #define LOG_CHECKPOINT_2 (3 * OS_FILE_LOG_BLOCK_SIZE)
704 /* second checkpoint field in the log
705 header */
706 #define LOG_FILE_HDR_SIZE (4 * OS_FILE_LOG_BLOCK_SIZE)
580 /* Offsets of a log block header */
581 #define LOG_BLOCK_HDR_NO 0 /* block number which must be > 0 and
582 is allowed to wrap around at 2G; the
583 highest bit is set to 1 if this is the
584 first log block in a log flush write
585 segment */
586 #define LOG_BLOCK_FLUSH_BIT_MASK 0x80000000UL
587 /* mask used to get the highest bit in
588 the preceding field */
589 #define LOG_BLOCK_HDR_DATA_LEN 4 /* number of bytes of log written to
590 this block */
591 #define LOG_BLOCK_FIRST_REC_GROUP 6 /* offset of the first start of an
592 mtr log record group in this log block,
593 0 if none; if the value is the same
594 as LOG_BLOCK_HDR_DATA_LEN, it means
595 that the first rec group has not yet
596 been catenated to this log block, but
597 if it will, it will start at this
598 offset; an archive recovery can
599 start parsing the log records starting
600 from this offset in this log block,
601 if value not 0 */
602 #define LOG_BLOCK_CHECKPOINT_NO 8 /* 4 lower bytes of the value of
603 log_sys->next_checkpoint_no when the
604 log block was last written to: if the
605 block has not yet been written full,
606 this value is only updated before a
607 log buffer flush */
608 #define LOG_BLOCK_HDR_SIZE 12 /* size of the log block header in
609 bytes */
610
611 /* Offsets of a log block trailer from the end of the block */
612 #define LOG_BLOCK_CHECKSUM 4 /* 4 byte checksum of the log block
613 contents; in InnoDB versions
614 < 3.23.52 this did not contain the
615 checksum but the same value as
616 .._HDR_NO */
617 #define LOG_BLOCK_TRL_SIZE 4 /* trailer size in bytes */
log_sys->lsn | 接下来将要生成的log record使用此lsn的值 |
log_sys->flushed_do_disk_lsn
|
redo log file已经被刷新到此lsn。比该lsn值小的日志记录已经被安全的记录在磁盘上 |
log_sys->write_lsn
|
当前正在执行的写操作使用的临界lsn值; |
log_sys->current_flush_lsn
|
当前正在执行的write + flush操作使用的临界lsn值,一般和log_sys->write_lsn相等; |
log_sys->buf
|
内存中全局的log buffer,和每个mtr自己的buffer有所区别;
|
log_sys->buf_size
|
log_sys->buf的size
|
log_sys->buf_free
|
写入buffer的起始偏移量
|
log_sys->buf_next_to_write
|
buffer中还未写到log file的起始偏移量。下次执行write+flush操作时,将会从此偏移量开始
|
log_sys->max_buf_free
|
确定flush操作执行的时间点,当log_sys->buf_free比此值大时需要执行flush操作,具体看log_check_margins函数
|
lsn是联系dirty page,redo log record和redo log file的纽带。在每个redo log record被拷贝到内存的log buffer时会产生一个相关联的lsn,而每个页面修改时会产生一个log record,从而每个数据库的page也会有一个相关联的lsn,这个lsn记录在每个page的header字段中。为了保证WAL(Write-Ahead-Logging)要求的逻辑,dirty page要求其关联lsn的log record已经被写入log file才允许执行flush操作。
376 /* Mini-transaction handle and buffer */
377 struct mtr_t{
378 #ifdef UNIV_DEBUG
379 ulint state; /*!< MTR_ACTIVE, MTR_COMMITTING, MTR_COMMITTED */
380 #endif
381 dyn_array_t memo; /*!< memo stack for locks etc. */
382 dyn_array_t log; /*!< mini-transaction log */
383 unsigned inside_ibuf:1;
384 /*!< TRUE if inside ibuf changes */
385 unsigned modifications:1;
386 /*!< TRUE if the mini-transaction
387 modified buffer pool pages */
388 unsigned made_dirty:1;
389 /*!< TRUE if mtr has made at least
390 one buffer pool page dirty */
391 ulint n_log_recs;
392 /* count of how many page initial log records
393 have been written to the mtr log */
394 ulint n_freed_pages;
395 /* number of pages that have been freed in
396 this mini-transaction */
397 ulint log_mode; /* specifies which operations should be
398 logged; default value MTR_LOG_ALL */
399 lsn_t start_lsn;/* start lsn of the possible log entry for
400 this mtr */
401 lsn_t end_lsn;/* end lsn of the possible log entry for
402 this mtr */
403 #ifdef UNIV_DEBUG
404 ulint magic_n;
405 #endif /* UNIV_DEBUG */
406 };
(trx_commit_in_memory() /
trx_commit_complete_for_mysql() /
trx_prepare() e.t.c)->
trx_flush_log_if_needed()->
trx_flush_log_if_needed_low()->
log_write_up_to()->
log_group_write_buf().
srv_master_thread()->
(srv_master_do_active_tasks() / srv_master_do_idle_tasks() / srv_master_do_shutdown_tasks())->
srv_sync_log_buffer_in_background()->
log_buffer_sync_in_background()->log_write_up_to()->... .
146 /** Wrapper for recv_recovery_from_checkpoint_start_func().
147 Recovers from a checkpoint. When this function returns, the database is able
148 to start processing of new user transactions, but the function
149 recv_recovery_from_checkpoint_finish should be called later to complete
150 the recovery and free the resources used in it.
151 @param type in: LOG_CHECKPOINT or LOG_ARCHIVE
152 @param lim in: recover up to this log sequence number if possible
153 @param min in: minimum flushed log sequence number from data files
154 @param max in: maximum flushed log sequence number from data files
155 @return error code or DB_SUCCESS */
156 # define recv_recovery_from_checkpoint_start(type,lim,min,max) \
157 recv_recovery_from_checkpoint_start_func(type,lim,min,max)
recv_sys->limit_lsn
|
恢复应该执行到的最大的LSN值,这里赋值为LSN_MAX(uint64_t的最大值) |
recv_sys->parse_start_lsn
|
恢复解析日志阶段所使用的最起始的LSN值,这里等于最后一次执行checkpoint对应的LSN值 |
recv_sys->scanned_lsn
|
当前扫描到的LSN值 |
recv_sys->recovered_lsn
|
当前恢复到的LSN值,此值小于等于recv_sys->scanned_lsn |
parse_start_lsn值是recovery的起点,其通过recv_find_max_checkpoint函数获取,读取的就是log文件LOG_CHECKPOINT_1/LOG_CHECKPOINT_2字段的值。
2908 /*******************************************************//**
2909 Scans log from a buffer and stores new log data to the parsing buffer. Parses
2910 and hashes the log records if new data found. */
2911 static
2912 void
2913 recv_group_scan_log_recs(
2914 /*=====================*/
2915 log_group_t* group, /*!< in: log group */
2916 lsn_t* contiguous_lsn, /*!< in/out: it is known that all log
2917 groups contain contiguous log data up
2918 to this lsn */
2919 lsn_t* group_scanned_lsn)/*!< out: scanning succeeded up to
2920 this lsn */ 2930 while (!finished) {
2931 end_lsn = start_lsn + RECV_SCAN_SIZE;
2932
2933 log_group_read_log_seg(LOG_RECOVER, log_sys->buf,
2934 group, start_lsn, end_lsn);
2935
2936 finished = recv_scan_log_recs(
2937 (buf_pool_get_n_pages()
2938 - (recv_n_pool_free_frames * srv_buf_pool_instances))
2939 * UNIV_PAGE_SIZE,
2940 TRUE, log_sys->buf, RECV_SCAN_SIZE,
2941 start_lsn, contiguous_lsn, group_scanned_lsn);
2942 start_lsn = end_lsn;
2943 }
105 /** Wrapper for recv_recover_page_func().
106 Applies the hashed log records to the page, if the page lsn is less than the
107 lsn of a log record. This can be called when a buffer page has just been
108 read in, or also for a page already in the buffer pool.
109 @param jri in: TRUE if just read in (the i/o handler calls this for
110 a freshly read page)
111 @param block in/out: the buffer block
112 */
113 # define recv_recover_page(jri, block) recv_recover_page_func(jri, block)
A: Log_file由一组log block组成,每个log block都是固定大小的。log block中除了header\tailer以外的字节都是记录的log record
2. Q: 是不是每一次的Commit,产生的应该是一个Log_block ?
A: 这个不一定的。写入log_block由mtr_commit确定,而不是事务提交确定。看log record大小,如果大小不需要跨log block,就会继续在当前的log block中写 。
3. Q: Log_record的结构又是怎么样的呢?
A: 这个结构很多,也没有细研究,具体看后边登博图中的简要介绍吧;
4. Q: 每个Block应该有下一个Block的偏移吗,还是顺序即可,还是记录下一个的Block_number
A: block都是固定大小的,顺序写的
5. Q: 那如何知道这个Block是不是完整的,是不是依赖下一个Block呢?
A: block开始有2个字节记录 此block中第一个mtr开始的位置,如果这个值是0,证明还是上一个block的同一个mtr。
6. Q: 一个事务是不是需要多个mtr_commit
A: 是的。mtr的m == mini;
7. Q: 这些Log_block是不是在Commit的时候一起刷到当中?
A: mtr_commit时会写入log buffer,具体什么时候写到log file就不一定了
8. Q: 那LSN是如何写的呢?
A: lsn就是相当于在log file中的位置,那么在写入log buffer时就会确定这个lsn的大小了 。当前只有一个log buffer,在log buffer中的位置和在log file中的位置是一致的
9. Q: 那我Commit的时候做什么事情呢?
A: 可能写log 、也可能不写,由innodb_flush_log_at_trx_commit这个参数决定啊
10. Q: 这两个值是干嘛用的: LOG_CHECKPOINT_1/LOG_CHECKPOINT_2
A: 这两个可以理解为log file头信息的一部分(占用文件头第二和第四个block),每次执行checkpoint都需要更新这两个字段,后续恢复时,每个页面对应lsn中比这个checkpoint值小的,认为是已经写入了,不需要再恢复
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