The redo log is a disk-based data structure used during crash recovery to correct data written by incomplete transactions. During normal operations, the redo log encodes requests to change table data that result from SQL statements or low-level API calls. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically during initialization, and before connections are accepted. For information about the role of the redo log in crash recovery, see Section 14.18.2, “InnoDB Recovery”.
By default, the redo log is physically represented on disk by two
ib_logfile1. MySQL writes to the redo log
files in a circular fashion. Data in the redo log is encoded in
terms of records affected; this data is collectively referred to
as redo. The passage of data through the redo log is represented
by an ever-increasing LSN value.
Information and procedures related to redo logs are described under the following topics in the section:
To change the number or size of
redo log files in MySQL
5.6.7 or earlier, perform the following steps:
mysql> SET GLOBAL innodb_fast_shutdown = 1;
After ensuring that
innodb_fast_shutdownis not set to 2, stop the MySQL server and make sure that it shuts down without errors (to ensure that there is no information for outstanding transactions in the log).
Copy the old log files into a safe place in case something went wrong during the shutdown and you need them to recover the tablespace.
Delete the old log files from the log file directory.
my.cnfto change the log file configuration.
setting is no longer relevant when changing the number or the
InnoDB log files. Additionally, you
are no longer required to remove old log files, although you may
still want to copy the old log files to a safe place, as a
backup. To change the number or size of
InnoDB log files, perform the following
InnoDB detects that the
from the redo log file size, it writes a log checkpoint, closes
and removes the old log files, creates new log files at the
requested size, and opens the new log files.
InnoDB, like any other
ACID-compliant database engine,
flushes the redo log of a
transaction before it is committed.
uses group commit
functionality to group multiple flush requests together to avoid
one flush for each commit. With group commit,
InnoDB issues a single write to the log file
to perform the commit action for multiple user transactions that
commit at about the same time, significantly improving