What is the difference between binlog/redolog/undolog in MySQL?
What is the difference between MySQL binlog/redolog/undolog?
If I want to talk to you about the locking mechanism in InnoDB, it will inevitably involve the MySQL log system, binlog, redo log, undo log, etc. I saw these three logs summarized by some friends. Not bad, hurry up and share it with your friends.
The log is an important part of the mysql
database, recording various status information during the operation of the database. mysql
Logs mainly include error logs, query logs, slow query logs, transaction logs, and binary logs.
As a developer, what we need to focus on is the binary log (binlog
) and transaction log (including redo log
and undo log
). This article will introduce these three types of logs in detail next.
bin log
binlog
is used to record write operations (excluding queries) information performed by the database and is saved on the disk in binary form. binlog
is the logical log of mysql
and is recorded by the Server
layer. The mysql
database using any storage engine will record binlog
Log.
Logical log: It can be understood that what is recorded is the sql statement.
Physical log:
mysql
The data is ultimately saved In the data page, the physical log records the data page changes.
binlog
is written by appending. Each binlog
file can be set through the max_binlog_size
parameter. When the file size reaches the given value, a new file will be generated to save the log.
In actual applications, binlog
has two main usage scenarios, namely master-slave replication and data recovery.
Master-slave replication: Open
binlog
on theMaster
side, and then sendbinlog
to eachSlave
side,Slave
side replaysbinlog
to achieve master-slave data consistency.Data recovery: Recover data by using the
mysqlbinlog
tool.
binlog flushing timing
For the InnoDB
storage engine, it will only be recorded when the transaction is committedbiglog
, the record is still in the memory at this time, so when was biglog
flushed to the disk?
mysql
Control the flushing timing of biglog
through the sync_binlog
parameter, the value range is 0-N
:
0: No mandatory requirement, the system will decide when to write to the disk;
1: Every time
commit
##binlogmust be written to the disk when #;
- N:
binlog
will be written to the disk for every N transactions.
sync_binlog is
1, which is also
MySQL 5.7.7Default value for subsequent versions. However, setting a larger value can improve database performance. Therefore, in actual situations, you can also increase the value appropriately and sacrifice a certain degree of consistency to obtain better performance.
binlogThe log has three formats, namely
STATMENT,
ROW and
MIXED.
MySQL 5.7.7, the default format is
STATEMENT, after
MySQL 5.7.7, the default value is
ROW . The log format is specified by
binlog-format.
STATMENT
: Replication based on
SQLstatements (
statement-based replication, SBR), each statement will be modified The SQL statements of the data will be recorded in
binlog.
ROW
: Row-based replication (
row-based replication, RBR), does not record the context information of each SQL statement, only It is necessary to record which data has been modified.
MIXED
: Mixed-based replication based on
STATMENTand
ROW, MBR
), general copying usesSTATEMENT
mode to savebinlog
, for operations that cannot be copied inSTATEMENT
mode, useROW
mode Savebinlog
redo log
Why do we need redo log
We all know the four major characteristics of transactions One of them is persistence. Specifically, as long as the transaction is submitted successfully, the modifications made to the database will be permanently saved, and it is impossible to return to the original state for any reason.So how does
mysqlensure consistency?
The simple way is to flush all data pages involved in modifications to disk every time a transaction is committed. However, doing so will cause serious performance problems, mainly reflected in two aspects:
Because
Innodb
performs disk interaction in units ofpages
, and a transaction is likely to only modify a few data pages. Bytes, it would be a waste of resources to flush the complete data page to the disk at this time!A transaction may involve modifying multiple data pages, and these data pages are not physically continuous. The performance of using random IO writing is too poor!
So
mysql
designedredo log
. Specifically, it only records what modifications the transaction has made to the data page, so It can perfectly solve the performance problem (relatively speaking, the file is smaller and it is sequential IO).
Basic concepts of redo log
redo log
includes two parts: one is the log buffer in memory (redo log buffer
), The other is the log file on disk (redo logfile
).
mysql
Each time a DML
statement is executed, the record is first written to redo log buffer
, and then at a later point in time, multiple records are written at once. Operation records are written to redo log file
. This technology of writing logs first and then writing to disk is the WAL (Write-Ahead Logging)
technology often mentioned in
MySQL.
In computer operating systems, buffer data in user space (user space
) generally cannot be written directly to the disk, and must pass through the operating system kernel space (kernel space
) buffer (OS Buffer
).
Therefore, redo log buffer
writing redo logfile
actually writes OS Buffer
first, and then calls through the system fsync()
Flash it to redo log file
, the process is as follows:
mysql
Support Three timings for writing redo log buffer
to redo log file
can be configured through the innodb_flush_log_at_trx_commit
parameter. The meaning of each parameter value is as follows:
redo log recording format
As mentioned earlier, redo log
actually records changes to the data page, and this It is not necessary to save all the change records, so redo log
is implemented using a fixed size and cyclic writing method. When writing to the end, it will return to the beginning to write the log in a loop. As shown below:
At the same time, we can easily know that in innodb, there are redo log
that need to be flushed, and also There are data pages
that also need to be flushed. The main significance of redo log
is to reduce the requirement for data pages
to be flushed**.
In the above figure, write pos
represents the LSN
(logical sequence number) position of the redo log
current record, check point
indicates the LSN
(logical sequence number) position corresponding to redo log
after the data page change record is flushed.
write pos
The part between check point
is the empty part of redo log
, used to record new records;## Between #check point and
write pos is the
redo log change record of the data page to be written to the disk. When
write pos catches up with
check point, it will first push
check point forward, vacate the position, and then record a new log.
innodb, no matter whether it was shut down normally or abnormally last time, the recovery operation will always be performed. Because
redo log records the physical changes of the data page, the recovery speed is much faster than the logical log (such as
binlog).
innodb, it will first check the
LSN of the data page in the disk. If the
LSN of the data page is less than the
in the log LSN, the recovery will start from
checkpoint.
checkpoint is in progress before the downtime, and the disk brushing progress of the data page exceeds the disk brushing progress of the log page. At this time, data will appear. The
LSN recorded in the page is greater than the
LSN in the log. At this time, the part that exceeds the progress of the log will not be redone, because this itself means that what has been done does not need to be redone. Do.
binlog and
redo log:
The binlog log is only used for archiving, and relying solely on
binlog does not have the
crash-safe capability.
But only redo log
will not work, because redo log
is unique to InnoDB
, and the records in the log will be overwritten after being written to disk. Therefore, both binlog
and redo log
need to be recorded at the same time to ensure that when the database is down and restarted, the data will not be lost.
undo log
One of the four major characteristics of database transactions is atomicity. Specifically, atomicity refers to a series of operations on the database, either all succeed or all fail. There may be partial success.
In fact, the bottom layer of atomicity is achieved through undo log
. undo log
mainly records the logical changes of data. For example, an INSERT
statement corresponds to a undo log
of DELETE
, for each # The ##UPDATE statement corresponds to an opposite
UPDATE's
undo log, so that when an error occurs, you can roll back to the data state before the transaction.
undo log is also the key to
MVCC (multi-version concurrency control) implementation.
The above is the detailed content of What is the difference between binlog/redolog/undolog in MySQL?. For more information, please follow other related articles on the PHP Chinese website!

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