Redo Logs -
Instead of writing to one file (the permanent table records file) you are writing to two different files (the redo log file and the permanent table records file). The difference is that the writes to the database table file(s) are random in nature while the writes to the redo log file will be sequential, which is usually going to be much faster. You perform the much-faster writes to the redo log as needed and then perform the slower writes to the table files periodically when there is time. Thus, the system actually operates faster writing to both files rather than only one file. This applying of the redo logs occurs automatically on startup of the MySQL server after a crash. Very similar processes are used by other database platforms.
Undo Logs -
In addition to the redo log there must also be undo logs. When a database user starts a transaction and executes some commands, the database does not know if the user will end the transaction with a
If the server crashed, the redo log files are applied first, on start up. This puts the database in a consistent state. Now the database server will need to roll back the transactions which were not committed but had already made changes to the database. Undo logs are used for this. As an example, if you are running a transaction that adds a million rows and the server crashes after eight hundred thousand inserts are performed, the server will first use the redo log to get the database server into a consistent stand than then will perform a rollback of the eight hundred thousand inserts using the undo logs. For InnoDB, this undo information is stored in the ibdata file(s).
Refer -
Instead of writing to one file (the permanent table records file) you are writing to two different files (the redo log file and the permanent table records file). The difference is that the writes to the database table file(s) are random in nature while the writes to the redo log file will be sequential, which is usually going to be much faster. You perform the much-faster writes to the redo log as needed and then perform the slower writes to the table files periodically when there is time. Thus, the system actually operates faster writing to both files rather than only one file. This applying of the redo logs occurs automatically on startup of the MySQL server after a crash. Very similar processes are used by other database platforms.
Undo Logs -
In addition to the redo log there must also be undo logs. When a database user starts a transaction and executes some commands, the database does not know if the user will end the transaction with a
COMMIT or a ROLLBACK command. Ending with a COMMIT
means all the changes made in the course of the transaction have to be
preserved (fulfilling the Durable aspect of ACID). If the transaction
gets interrupted for some reason, such as the MySQL daemon crashing, the
client disconnecting before sending a COMMIT, or the user issuing a ROLLBACK command, then all changes made by the transaction need to be undone.If the server crashed, the redo log files are applied first, on start up. This puts the database in a consistent state. Now the database server will need to roll back the transactions which were not committed but had already made changes to the database. Undo logs are used for this. As an example, if you are running a transaction that adds a million rows and the server crashes after eight hundred thousand inserts are performed, the server will first use the redo log to get the database server into a consistent stand than then will perform a rollback of the eight hundred thousand inserts using the undo logs. For InnoDB, this undo information is stored in the ibdata file(s).
Refer -
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