事务通过trx_rsegs_t与系统表空间和临时表空间等物理存储联系起来的方式如下：
/** Rollback segments assigned to a transaction for undo logging. */
struct trx_rsegs_t {
/** undo log ptr holding reference to a rollback segment that resides in
system/undo tablespace used for undo logging of tables that needs to be recovered on crash. */
trx_undo_ptr_t m_redo; //系统的UNDO表空间
/** undo log ptr holding reference to a rollback segment that resides in
temp tablespace used for undo logging of tables that doesn't need to be recovered on crash. */
trx_undo_ptr_t m_noredo; //系统的临时表空间
};
系统表空间和临时表空间等物理存储与UNDO日志之间的关系如下：
/** Represents an instance of rollback segment along with its state variables.*/
struct trx_undo_ptr_t {
//标识分配给事务的回滚段，这样把事务和回滚段建立起联系来。然后通过trx_rsegs_t与系统表空间和临时表空间等物理存储联系起来
trx_rseg_t* rseg; //指向回滚段
trx_undo_t* insert_undo; /*!< pointer to the insert undo log, or NULL if no inserts performed yet */ //事务指向insert undo log
trx_undo_t* update_undo; /*!< pointer to the update undo log, or ULL if no update performed yet */ //事务指向update undo log
};
而回滚段的信息又如下：
/** The rollback segment memory object */
struct trx_rseg_t {
ulint id; //回滚段的标识
...
ulint space; //回滚段的头信息在表空间中的位置，表空间标识
ulint page_no; //回滚段的头信息在表空间中的位置，页号
page_size_t page_size; /** page size of the relevant tablespace */
ulint max_size; /** maximum allowed size in pages */
ulint curr_size; /** current size in pages */
...
/** 执行UPDATE操作产生的UODO日志，包括先删除后插入的过程中产生的UODO信息，事务完成，信息依然被保留，用于MVCC机制下的一致性读 */
/** List of update undo logs */
UT_LIST_BASE_NODE_T(trx_undo_t) update_undo_list;
/** List of update undo log segments cached for fast reuse */
UT_LIST_BASE_NODE_T(trx_undo_t) update_undo_cached;
/* 执行INSERT操作产生的UODO日志，这些信息是临时的，事务结束后就被清理 */
/** List of insert undo logs */
UT_LIST_BASE_NODE_T(trx_undo_t) insert_undo_list;
/** List of insert undo log segments cached for fast reuse */
UT_LIST_BASE_NODE_T(trx_undo_t) insert_undo_cached;
...
};