本篇内容介绍了“Innodb undo结构是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
rollback segments(128)
undo segments(1024)
undo log (header insert/modify 分开的) <-> undo page
undo record
undo record
作为undo segments的第一个undo page可以存放多个事物的undo log，因为如果这个块的undo 记录没有填满3/4则会进入 rollback segment的cache list,那么下次可以继续使用，但是如果第一个块不足以装下事物的undo 记录，那么很显然需要分配新的undo page，这种情况下一个undo page就只能包含一个事物的undo记录了。
事物每次需要分配rollback segments然后分配undo segments然后初始化好undo log header，insert和update/delete需要分配不同的undo segments，一个undo segments往往对应了一个undo log，undo log可以包含多个undo record(因为从debug来看undo log header的初始化只做了一次)，对于操作的每行都会留下一个undo record作为mvcc构建历史版本的基础。
undo生成的基本单位是undo record，每行记录都会包含一个undo record，而rollback ptr指向的是undo record的偏移量，对于每行的记录都会去判断其可见性，如果需要构建前版本则通过本指针进行构建包含：
第1位是否是insert 第2到8位是undo segment id 第9到40位为page no 第41位到56位为 offset
每一个undo log包含一个trx_undo_t结构体
每一个rollback segments包含一个trx_rseg_t结构体
undo page header 每一个undo page都包含
/** Transaction undo log page header offsets *//* @{ */#define TRX_UNDO_PAGE_TYPE 0 /*!< TRX_UNDO_INSERT or
TRX_UNDO_UPDATE */#define TRX_UNDO_PAGE_START 2 /*!< Byte offset where the undo log
records for the LATEST transaction
start on this page (remember that
in an update undo log, the first page
can contain several undo logs) */#define TRX_UNDO_PAGE_FREE 4 /*!< On each page of the undo log this
field contains the byte offset of the
first free byte on the page */#define TRX_UNDO_PAGE_NODE 6 /*!< The file list node in the chain
of undo log pages */undo semgent header 第一个page 才会用 undo segment header信息
#define TRX_UNDO_STATE 0 /*!< TRX_UNDO_ACTIVE, ... */#ifndef UNIV_INNOCHECKSUM#define TRX_UNDO_LAST_LOG 2 /*!< Offset of the last undo log header on the segment header page, 0 if none */#define TRX_UNDO_FSEG_HEADER 4 /*!< Header for the file segment which the undo log segment occupies */#define TRX_UNDO_PAGE_LIST (4 + FSEG_HEADER_SIZE) /*!< Base node for the list of pages in the undo log segment; defined only on the undo log segment's first page */
每一个undo log
undo log header
undo log record 相应的undo实际内容
undo log record 相应的undo实际内容
undo log header 包含
#define TRX_UNDO_TRX_ID 0 /*!< Transaction id */#define TRX_UNDO_TRX_NO 8 /*!< Transaction number of the transaction; defined only if the log is in a history list */#define TRX_UNDO_DEL_MARKS 16 /*!< Defined only in an update undo log: TRUE if the transaction may have done delete markings of records, and thus purge is necessary */#define TRX_UNDO_LOG_START 18 /*!< Offset of the first undo log record of this log on the header page; purge may remove undo log record from the log start, and therefore this is not necessarily the same as this log header end offset */#define TRX_UNDO_XID_EXISTS 20 /*!< TRUE if undo log header includes X/Open XA transaction identification XID */#define TRX_UNDO_DICT_TRANS 21 /*!< TRUE if the transaction is a table create, index create, or drop transaction: in recovery the transaction cannot be rolled back in the usual way: a 'rollback' rather means dropping the created or dropped table, if it still exists */#define TRX_UNDO_TABLE_ID 22 /*!< Id of the table if the preceding field is TRUE */#define TRX_UNDO_NEXT_LOG 30 /*!< Offset of the next undo log header on this page, 0 if none */#define TRX_UNDO_PREV_LOG 32 /*!< Offset of the previous undo log header on this page, 0 if none */#define TRX_UNDO_HISTORY_NODE 34 /*!< If the log is put to the history list, the file list node is here */
第一步为 分配rollback segments
#0 get_next_redo_rseg (max_undo_logs=128, n_tablespaces=4) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/trx/trx0trx.cc:1138#1 0x0000000001c0bce8 in trx_assign_rseg_low (max_undo_logs=128, n_tablespaces=4, rseg_type=TRX_RSEG_TYPE_REDO) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/trx/trx0trx.cc:1314#2 0x0000000001c1097d in trx_set_rw_mode (trx=0x7fffd7804080) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/trx/trx0trx.cc:3352#3 0x0000000001a64013 in lock_table (flags=0, table=0x7ffeac012ae0, mode=LOCK_IX, thr=0x7ffe7c92ef48) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:4139#4 0x0000000001b7950e in row_search_mvcc (buf=0x7ffe7c92e350 "\377", mode=PAGE_CUR_GE, prebuilt=0x7ffe7c92e7d0, match_mode=1, direction=0) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:5100#5 0x00000000019d5443 in ha_innobase::index_read (this=0x7ffe7c92de10, buf=0x7ffe7c92e350 "\377", key_ptr=0x7ffe7cd57590 "\004", key_len=4, find_flag=HA_READ_KEY_EXACT) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:9536#6 0x0000000000f9345a in handler::index_read_map (this=0x7ffe7c92de10, buf=0x7ffe7c92e350 "\377", key=0x7ffe7cd57590 "\004", keypart_map=1, find_flag=HA_READ_KEY_EXACT) at /root/mysqlc/percona-server-locks-detail-5.7.22/sql/handler.h:2942#7 0x0000000000f83e44 in handler::ha_index_read_map (this=0x7ffe7c92de10, buf=0x7ffe7c92e350 "\377", key=0x7ffe7cd57590 "\004", keypart_map=1, find_flag=HA_READ_KEY_EXACT) at /root/mysqlc/percona-server-locks-detail-5.7.22/sql/handler.cc:3248
第二步 对于主键每行更改操作都会调用trx_undo_report_row_operation 他会分配undo segments 并且会负责写入undo record
#0 trx_undo_report_row_operation (flags=0, op_type=2, thr=0x7ffe7c932828, index=0x7ffea4016590, clust_entry=0x7ffe7c932cc0, update=0x0, cmpl_info=0, rec=0x7fffb580d369 "", offsets=0x7fffec0f3e00, roll_ptr=0x7fffec0f3688) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/trx/trx0rec.cc:1866#1 0x0000000001c5795b in btr_cur_del_mark_set_clust_rec (flags=0, block=0x7fffb4ccaae0, rec=0x7fffb580d369 "", index=0x7ffea4016590, offsets=0x7fffec0f3e00, thr=0x7ffe7c932828, entry=0x7ffe7c932cc0, mtr=0x7fffec0f38f0) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/btr/btr0cur.cc:4894#2 0x0000000001b9f218 in row_upd_del_mark_clust_rec (flags=0, node=0x7ffe7c932550, index=0x7ffea4016590, offsets=0x7fffec0f3e00, thr=0x7ffe7c932828, referenced=0, mtr=0x7fffec0f38f0) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2778#3 0x0000000001b9f765 in row_upd_clust_step (node=0x7ffe7c932550, thr=0x7ffe7c932828) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2923#4 0x0000000001b9fc74 in row_upd (node=0x7ffe7c932550, thr=0x7ffe7c932828) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:3042#5 0x0000000001ba0155 in row_upd_step (thr=0x7ffe7c932828) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:3188#6 0x0000000001b3d3a0 in row_update_for_mysql_using_upd_graph (mysql_rec=0x7ffe7c9318d0 "\375\001", prebuilt=0x7ffe7c931d50) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0mysql.cc:3040#7 0x0000000001b3d6a1 in row_update_for_mysql (mysql_rec=0x7ffe7c9318d0 "\375\001", prebuilt=0x7ffe7c931d50) at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/row/row0mysql.cc:3131#8 0x00000000019d47c3 in ha_innobase::delete_row (this=0x7ffe7c931390, record=0x7ffe7c9318d0 "\375\001") at /root/mysqlc/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:9141
大概流程
switch (op_type)
{ case TRX_UNDO_INSERT_OP:
undo = undo_ptr->insert_undo; //如果是 insert 则使用insert_undo 类型为trx_undo_t 指针
if (undo == NULL) { //如果已经分配了就不用分配了
err = trx_undo_assign_undo( //分配undo segment 同时初始化 undo log header
trx, undo_ptr, TRX_UNDO_INSERT);
undo = undo_ptr->insert_undo;
...
} break; default:
ut_ad(op_type == TRX_UNDO_MODIFY_OP); //断言
undo = undo_ptr->update_undo; if (undo == NULL) {
err = trx_undo_assign_undo(
trx, undo_ptr, TRX_UNDO_UPDATE); //分配undo segment 同时初始化 undo log header
undo = undo_ptr->update_undo;
...
}
... case TRX_UNDO_INSERT_OP://注意是每行都会操作
offset = trx_undo_page_report_insert( //写入insert undo log record
undo_page, trx, index, clust_entry, &mtr); break; default:
ut_ad(op_type == TRX_UNDO_MODIFY_OP); //写入delete update undo log record
offset = trx_undo_page_report_modify(
undo_page, trx, index, rec, offsets, update,
cmpl_info, clust_entry, &mtr);
}
...
*roll_ptr = trx_undo_build_roll_ptr( //构建rollback ptr 主键中每行都有这个 用于MVCC构建回滚版本
op_type == TRX_UNDO_INSERT_OP,
undo_ptr->rseg->id, page_no, offset);我将undo log record的写入到了错误日志，下面进行简单的分解。
表结构如下：
mysql> show create table t1; +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------+ | t1 | CREATE TABLE `t1` ( `id1` int(11) NOT NULL, `id2` int(11) DEFAULT NULL, PRIMARY KEY (`id1`), KEY `id2` (`id2`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)
insert 的undo记录，具体构造在trx_undo_page_report_insert中
语句
mysql> insert into t1 values(28,28); Query OK, 1 row affected (0.00 sec)
输出如下：
trx_undo_assign_undo:assign undo space: RSEG SLOT:34,RSEG SPACE ID:2 PAGE NO:3UNDO SLOT:0,UNDO SPACE ID:2 UNDO LOG HEADER PAGE NO:27,UNDO LOG HEADER OFFSET:86,UNDO LOG LAST PAGE:27trx_undo_page_report_insert:undo log record TABLE_NAME:test/t1 TRX_ID:12591,UODO RECORD LEN:10 len 10; hex 011e0b0032048000001c;
011e0b0032048000001c就是undo record的实际记录解析如下：
011c page内部本undo record结束的位置0b 类型为 #define TRX_UNDO_INSERT_REC 11(0X0b)00 undo no,提交才会有32 table_id 可以查询 INNODB_SYS_TABLES 对照04 字段长度4个字节8000001c 我插入的记录主键 28(0X1c)
update 的undo记录，具体构造在trx_undo_page_report_modify中
语句：
mysql> update t1 set id2=1000 where id1=14; Query OK, 1 row affected (5 min 40.91 sec) Rows matched: 1 Changed: 1 Warnings: 0
输出如下：
trx_undo_assign_undo:assign undo space: RSEG SLOT:41,RSEG SPACE ID:1 PAGE NO:5UNDO SLOT:1,UNDO SPACE ID:1 UNDO LOG HEADER PAGE NO:37,UNDO LOG HEADER OFFSET:1389,UNDO LOG LAST PAGE:37trx_undo_page_report_modify:undo log record TABLE_NAME:test/t1 TRX_ID:12604,UODO RECORD LEN:47 len 47; hex 06560c0032000000003136e0260000002c052e048000000e010304800003e7000e00048000000e0304800003e70627;
06560c0032000000003136e0260000002c052e048000000e010304800003e7000e00048000000e030480
就是undo record的记录
大体解析如下：
0656 :page内部本undo record结束的位置 0c:类型为 #define TRX_UNDO_UPD_EXIST_REC 12(0X0c) 00: undo no,提交才会有 32: table_id 可以查询 INNODB_SYS_TABLES 对照 00: 0000003136e0:事物ID260000002c052e:undo回滚指针 04:主键长度 8000000e:主键值 01 03:位置 04:被修改值的长度 800003e7:值为999(0x3e7) 000e:接下来字符的长度，记录原始值？ 00:位置 04:长度 8000000e:主键值 03:位置 04:长度 800003e7:值为999(0x3e7) 0627:page内部本undo record开始的位置,0X0656-0X0627就是长度
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