Undo log details

Today I'd like to share with you Undo log Knowledge points of log

1. What is? undo log？

Undo log , Before the database transaction starts ,MYSQL Will record the data before the update to undo log In file . If the transaction rolls back or the database crashes , You can use undo log Log information recorded in the log is returned . At the same time, it can also provide reading under multi version concurrency control (MVCC).（ Concrete MVCC See this blog for a detailed explanation of the implementation mechanism ）

2.undo log Life cycle

undo log produce ： Generate before the transaction starts
undo log The destruction ： When the transaction is committed ,undo log It can't be deleted immediately , But put it in the list to be cleaned up , from purge The thread determines whether it is being used by other transactions undo Version information before the last transaction of the table in Section , Decide if you can clean up undo log The log space of .
Be careful ： undo log Will also produce redo log,undo log We also need to achieve durable protection .

3. uodo log The role of logs

First of all, briefly undolog and redo log The difference between
undo log It's a logical log , Realize the atomicity of transactions
  undo log Records are transactions [ Prior to the start ] Data status of , Record the value before update
  undo log Realize the atomicity of transactions ( Provide rollback )
redo log It's a physical log , Achieve transaction persistence
  undo log Records are transactions [ After completion ] Data status of , What is recorded is the updated value
  undo log Achieve transaction persistence ( Ensure data integrity )

1.undo log Logging can realize the rollback of transactions
When we update the data , Not only will it record redo log, And record undo log, If the transaction is rolled back for some reason , So at this point MySQL You have to roll back （rollback） operation , utilize undo log Restore the data to the state before the transaction started .

If we execute the following deletion statement ：

delete from book where id = 1;

So at this time undo log Will generate a contrary insert sentence 【 Reverse operation statement 】, When transaction rollback is required , Directly implement this article sql, The data can be completely restored to the data before modification , So as to achieve the purpose of transaction rollback .

Another example is that we execute a update sentence ：

update book set name = " three countries " where id = 1;   --- Before the change name= Journey to the west 

here undo log It's going to record the opposite update sentence , as follows ：

update book set name = " Journey to the west " where id = 1;

If the modification is abnormal , have access to undo log Log to implement rollback operation , To ensure the consistency of transactions .

2.undo log Realize multi version concurrency control MVCC
Concrete MVCC See this blog for a detailed explanation of the implementation mechanism

4. uodo log How it works

As shown in the figure above ：
When a transaction A Carry out a update operation , take id=1 Modified into id=2. First of all, it will change buffer pool Cache data in , At the same time, the old data will be backed up to undo log buffer in , What is recorded is the restore operation sql sentence . At this point, if the transaction B To query the modified data , But the business A Not yet submitted , The transaction B It will start from undo log buffer in , Query transaction A Modify the previous data , That is to say id=1. here undo log buffer Will persist data to undo log In the log ( Plate dropping operation ).undo After log persistence , Will actually write data to disk , That is to write ibd In the file of . Finally, the transaction submission will be executed .

5. uodo log The storage mechanism of

The specific parameter storage form is shown in the above figure

6. uodo log Configuration parameters for

innodb_max_undo_log_size: undo The maximum number of log files , Default 1GB, Initialization size 10M
innodb_undo_log_truncate: Identify whether auto shrink is on undo log Operation of table space
innodb_undo_tablespaces: Set the number of independent tablespaces , The default is 0, Flag does not open independent tablespaces ,undo The log is kept in ibdata1 in
innodb_undo_directory: undo Directory location of log storage
innodb_undo_logs: Number of rollback Default 128