that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option. See Also startup. 

low-water mark 

 A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast with high-water mark . See Also high-water mark. 

LRU 

 An acronym for “least recently used” , a common method for managing storage areas. The items that have not been used recently are evicted  when space is needed to cache newer items. InnoDB  uses the LRU mechanism by default to manage the pages  within the buffer pool , but makes exceptions in cases where a page might be read only a single time, such as during a full table scan . This variation of the LRU algorithm is called the midpoint insertion strategy . For more information, see Section 15.5.1, “Buffer Pool” . See Also buffer pool, eviction, full table scan, midpoint insertion strategy, page. 

LSN 

 Acronym for “log sequence number” . This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log . (This point in time is regardless of transaction  boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB  during crash recovery  and for managing the buffer pool . 

Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values. 

In the MySQL Enterprise Backup  product, you can specify an LSN to represent the point in time from which to take an incremental backup . The relevant LSN is displayed by the output of the mysqlbackup  command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup. See Also buffer pool, crash recovery, incremental backup, MySQL Enterprise Backup, redo log, transaction. 

## M 

.MRG file 

 A file containing references to other tables, used by the MERGE  storage engine. Files with this extension are always included in backups produced by the mysqlbackup  command of the MySQL Enterprise Backup product. See Also MySQL Enterprise Backup, mysqlbackup command. 

.MYD file 

 A file that MySQL uses to store data for a MyISAM  table. See Also .MYI file, MySQL Enterprise Backup, mysqlbackup command. 

.MYI file 

 A file that MySQL uses to store indexes for a MyISAM  table. See Also .MYD file, MySQL Enterprise Backup, mysqlbackup command. 

master 

See source. 

master thread 

 An InnoDB thread  that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the change buffer  to the appropriate secondary indexes. 

