- Any changes to a page is first done to the in-memory copy of the page. The page that is modified in memory and not yet flushed to disk is marked as the dirty page in memory.
- An associated redo log is generated in memory, in the local mini transaction (mtr) buffer. This will then be transferred to the global in-memory redo log buffer.
- The redo log record is written from the redo log buffer in memory to the redo log file on disk. This is subsequently flushed. These two steps are considered separate - writing to the redo log file and flushing the redo log file to the disk. This is to account for the file buffering done by the operating system.
- The dirty page is flushed from memory to disk at some later point of time as part of the checkpointing operation.
The order of these steps are important. The redo log record of a change must be flushed to disk before flushing the corresponding dirty page to the disk. This is the concept of write-ahead logging (WAL).