See Also compressed row format, compression. compression  A feature with wide-ranging benefits from using less disk space, performing less I/O, and using less memory for caching. InnoDB  supports both table-level and page-level compression.  InnoDB  page compression is also referred to as transparent page compression . For more information about  InnoDB  compression, see Section 15.9, “InnoDB Table and Page Compression” . Another type of compression is the  compressed backup  feature of the  MySQL Enterprise Backup  product. See Also buffer pool, compressed backup, compressed row format, DML, transparent page compression. compression failure  Not actually an error, rather an expensive operation that can occur when using  compression  in combination with  DML  operations. It occurs when: updates to a compressed  page  overflow the area on the page reserved for recording modifications; the page is compressed again, with all changes applied to the table data; the re-compressed data does not fit on the original page, requiring MySQL to split the data into two new pages and compress each one separately. To check the frequency of this condition, query the INFORMATION_SCHEMA.INNODB_CMP  table and check how much the value of the  COMPRESS_OPS  column exceeds the value of the  COMPRESS_OPS_OK  column. Ideally, compression failures do not occur often; when they do, you can adjust the  innodb_compression_level ,  innodb_compression_failure_threshold_pct , and  innodb_compression_pad_pct_max  configuration options. See Also compression, DML, page. concatenated index See composite index. concurrency  The ability of multiple operations (in database terminology,  transactions ) to run simultaneously, without interfering with each other. Concurrency is also involved with performance, because ideally the protection for multiple simultaneous transactions works with a minimum of performance overhead, using efficient mechanisms for  locking . See Also ACID, locking, transaction. configuration file  The file that holds the  option  values used by MySQL at startup. Traditionally, on Linux and Unix this file is named  my.cnf , and on Windows it is named  my.ini . You can set a number of options related to InnoDB under the  [mysqld]  section of the file. See Section 4.2.2.2, “Using Option Files”  for information about where MySQL searches for configuration files. When you use the  MySQL Enterprise Backup  product, you typically use two configuration files: one that specifies where the data comes from and how it is structured (which could be the original configuration file for your server), and a stripped-down one containing only a small set of options that specify where the backup data goes and how it is structured. The configuration files used with the  MySQL Enterprise Backup  product must contain certain options that are typically left out of regular configuration files, so you might need to add options to your existing configuration file for use with  MySQL Enterprise Backup . See Also my.cnf, MySQL Enterprise Backup, option, option file. connection  The communication channel between an application and a MySQL server. The performance and scalability of a database applications is influenced by on how quickly a database connection can be established, how many can be made simultaneously, and how long they persist. The parameters such as  host ,  port , and so on are represented as a  connection string  in  Connector/NET , and as a  DSN  in  Connector/ODBC . High-traffic systems make use of an optimization known as the  connection pool . 6142