我的学习记录，可能有误请谅解，提供了一些源码接口供有兴趣的朋友调试。
版本：5.7.17
一、LSN
innodb的lsn和oracle的scn一样，是一个重要的概念。是整个数据库数据同步的一种统一辨别标准，在很多地方都使用到了LSN比如
- 在flush list中正是是使用page的oldest lsn作为链表的条件
参考buf_page_t类中的 lsn_t oldest_modification;变量 - 在checkpoint中记录的也是lsn
参考宏 LOG_CHECKPOINT_LSN - 在物理文件中每个page最后的刷新lsn
参考宏FIL_PAGE_LSN - 在写日志落盘的时候也是以lsn为标准的
参考函数log_write_up_to
实际上lsn就是表示的日志量的字节数，是一个累加的值，在5.7中表现为:
/* Type used for all log sequence number storage and arithmetics */
typedef ib_uint64_t lsn_t;
及一个8字节非负的整数。最大值及2的64次方。有了这种物理上概念，lsn很容易换算为当前日志的偏移量。
二、innodb中检查点的理解
这里我只讨论正常运行的情况下检查点。innodb中类似oracle的增量检查点。正常运行checkpoint是由master线程触发。我们知道脏数据通过page clean线程和lru manager线程是在不断写盘的，那么在进行异常重启的的时候我们必须要知道一个恢复的起点，但是这个起点是不能记录在内存中必要固化到磁盘，恢复的时候读取这个点以后的redo进行恢复，而checkpoint就是完成这个事情下面是checkpoint的执行流程。
正常情况下master线程会每秒进行检查点其作用有(参考log_checkpoint函数)：
- 检查是否有自上次检查点以来的脏数据写盘了。
- 如果有则在redo里面会为每个修改过的文件写入MLOG_FILE_NAME,完成后写入一个总MLOG_CHECKPOINT(参考fil_names_clear函数)。
1、MLOG_FILE_NAME主要记录至上次检查点以来更改过的数据文件。
2、MLOG_CHECKPOINT主要记录检查点的lsn。
这个步骤会遍历fil_system->named_spaces用于查找是否有自上次检查点以来修改过的文件链表。 - 如果有则在redo log header中写入相应的检查点信息包含(异步写)。
实际上我们可以理解检查点就是由master线程每秒醒来查看一下脏数据写到哪里了，然后将其记录到合适的位置，以备carsh recovery使用。(参考srv_master_thread函数)
三、show engine innodb中的检查点信息
下面是一个没有任何更新操作的库的信息如下：
Log sequence number 697794162
Log flushed up to 697794162
Pages flushed up to 697794162
Last checkpoint at 697794153
- Log sequence number：已经写到log buffer中的lsn。
参考mtr_t::Command::finish_write函数。 - Log flushed up to：已经写到日志文件的redo的lsn。
参考log_write_flush_to_disk_low函数。 - Pages flushed up to ：此lsn之前的脏数据都已经写到了数据文件。
参考log_buf_pool_get_oldest_modification函数。 - Last checkpoint at ：最后一次检查点记录到了什么位置。
参考log_complete_checkpoint函数。
下面是这段输出的源码：
fprintf(file,
"Log sequence number " LSN_PF "\n"
"Log flushed up to " LSN_PF "\n"
"Pages flushed up to " LSN_PF "\n"
"Last checkpoint at " LSN_PF "\n",
log_sys->lsn,
log_sys->flushed_to_disk_lsn,
log_buf_pool_get_oldest_modification(),
log_sys->last_checkpoint_lsn);
一般来讲Log sequence number >Log flushed up to> Pages flushed up to>Last checkpoint at ，但是这里注意一下。Pages flushed up to 697794162和Last checkpoint at 697794153，显然这里是一个没有任何操作的库所以Pages flushed up to应该和Last checkpoint at 相等，但是这里存在差值，差值为：
- 697794162-697794153 = 9
这刚好是MLOG_CHECKPOINT的长度源码片段如下：
oldest_lsn <= log_sys->last_checkpoint_lsn + SIZE_OF_MLOG_CHECKPOINT
/** Size of a MLOG_CHECKPOINT record in bytes.
The record consists of a MLOG_CHECKPOINT byte followed by
mach_write_to_8(checkpoint_lsn). */
#define SIZE_OF_MLOG_CHECKPOINT 9
四、我所debug的几种redo写盘的时机
- master 线程每秒调用 栈帧(可能是idle可能是active 和检测是否需要插入缓存合并有关)
#0 log_group_write_buf (group=0x33f29f8, buf=0x7fffa5b38000 "\200\024", len=512, pad_len=0, start_lsn=697764864, new_data_offset=166)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1145
#1 0x0000000001a50f95 in log_write_up_to (lsn=697765068, flush_to_disk=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1493
#2 0x0000000001a51163 in log_buffer_sync_in_background (flush=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1553
#3 0x0000000001b84bd1 in srv_sync_log_buffer_in_background () at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/srv/srv0srv.cc:2312
#4 0x0000000001b85666 in srv_master_do_idle_tasks () at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/srv/srv0srv.cc:2586
#5 0x0000000001b85b6b in srv_master_thread (arg=0x0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/srv/srv0srv.cc:2744
- master 线程每秒checkpoint调用 (可能是idle可能是active 和检测是否需要插入缓存合并有关)
#0 log_group_write_buf (group=0x33f29f8, buf=0x7fffa5a38000 "\200\024\002", len=1024, pad_len=0, start_lsn=697789952, new_data_offset=139)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1145
#1 0x0000000001a50f95 in log_write_up_to (lsn=697790725, flush_to_disk=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1493
#2 0x0000000001a52247 in log_checkpoint (sync=true, write_always=false) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1934
#3 0x0000000001b856f2 in srv_master_do_idle_tasks () at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/srv/srv0srv.cc:2596
#4 0x0000000001b85b6b in srv_master_thread (arg=0x0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/srv/srv0srv.cc:2744
- page clean 线程调用 栈帧
#0 log_group_write_buf (group=0x33f29f8, buf=0x7fffa5a38000 "\200\024\002", len=13312, pad_len=1024, start_lsn=697778176, new_data_offset=468)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1145
#1 0x0000000001a50f95 in log_write_up_to (lsn=697790015, flush_to_disk=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1493
#2 0x0000000001c704c7 in buf_flush_write_block_low (bpage=0x7fffc0cae940, flush_type=BUF_FLUSH_LIST, sync=false)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/buf/buf0flu.cc:1035
#3 0x0000000001c70cea in buf_flush_page (buf_pool=0x33247d8, bpage=0x7fffc0cae940, flush_type=BUF_FLUSH_LIST, sync=false)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/buf/buf0flu.cc:1237
#4 0x0000000001c717f4 in buf_flush_try_neighbors (page_id=..., flush_type=BUF_FLUSH_LIST, n_flushed=0, n_to_flush=25)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/buf/buf0flu.cc:1466
- 当前线程commit 调用栈帧如下：
#0 log_group_write_buf (group=0x33f29f8, buf=0x7fffa5a38000 "\200\024\002", len=2560, pad_len=0, start_lsn=697762816, new_data_offset=230)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1145
#1 0x0000000001a50f95 in log_write_up_to (lsn=697765030, flush_to_disk=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1493
#2 0x0000000001a51087 in log_buffer_flush_to_disk (sync=true) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/log/log0log.cc:1524
#3 0x00000000019a9157 in innobase_flush_logs (hton=0x2e9fdd0, binlog_group_flush=true)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:4407
#4 0x0000000000f65893 in flush_handlerton (thd=0x0, plugin=0x7ffff03588e8, arg=0x7ffff0358944) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:2606
#5 0x00000000015d7716 in plugin_foreach_with_mask (thd=0x0, func=0xf65835 , type=1, state_mask=4294967287,
arg=0x7ffff0358944) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_plugin.cc:2318
#6 0x0000000000f658ef in ha_flush_logs (db_type=0x0, binlog_group_flush=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:2617
#7 0x000000000185733d in MYSQL_BIN_LOG::process_flush_stage_queue (this=0x2e02c80, total_bytes_var=0x7ffff0358a88, rotate_var=0x7ffff0358a87,
out_queue_var=0x7ffff0358a78) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8541
#8 0x000000000185899f in MYSQL_BIN_LOG::ordered_commit (this=0x2e02c80, thd=0x7fff2c000b70, all=false, skip_commit=false)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:9189
#9 0x000000000185700c in MYSQL_BIN_LOG::commit (this=0x2e02c80, thd=0x7fff2c000b70, all=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8440
#10 0x0000000000f63df8 in ha_commit_trans (thd=0x7fff2c000b70, all=false, ignore_global_read_lock=false)
- innodb shutdown(未debug)
- redo buffer不足(未debug)
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