MySQL Glossary A adaptive hash index adaptive flushing ACID .ARM file .ARZ file These terms are commonly used in information about the MySQL database server. This glossary originated as a reference for terminology about the InnoDB storage engine, and the majority of definitions are InnoDB-related.  Metadata for  ARCHIVE  tables. Contrast with  .ARZ file . Files with this extension are always included in backups produced by the  mysqlbackup  command of the  MySQL Enterprise Backup  product. See Also .ARZ file, MySQL Enterprise Backup, mysqlbackup command.  Data for ARCHIVE tables. Contrast with  .ARM file . Files with this extension are always included in backups produced by the  mysqlbackup  command of the  MySQL Enterprise Backup  product. See Also .ARM file, MySQL Enterprise Backup, mysqlbackup command.  An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable in a database system, and are all closely tied to the notion of a  transaction . The transactional features of  InnoDB adhere to the ACID principles. Transactions are  atomic  units of work that can be  committed  or  rolled back . When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back. The database remains in a consistent state at all times — after each commit or rollback, and while transactions are in progress. If related data is being updated across multiple tables, queries see either all old values or all new values, not a mix of old and new values. Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with each other or see each other’s uncommitted data. This isolation is achieved through the  locking  mechanism. Experienced users can adjust the  isolation level , trading off less protection in favor of increased performance and  concurrency , when they can be sure that the transactions really do not interfere with each other. The results of transactions are durable: once a commit operation succeeds, the changes made by that transaction are safe from power failures, system crashes, race conditions, or other potential dangers that many non-database applications are vulnerable to. Durability typically involves writing to disk storage, with a certain amount of redundancy to protect against power failures or software crashes during write operations. (In  InnoDB , the doublewrite buffer  assists with durability.) See Also atomic, commit, concurrency, doublewrite buffer, isolation level, locking, rollback, transaction.  An algorithm for  InnoDB  tables that smooths out the I/O overhead introduced by  checkpoints . Instead of flushing  all modified  pages  from the  buffer pool  to the  data files  at once, MySQL periodically flushes small sets of modified pages. The adaptive flushing algorithm extends this process by estimating the optimal rate to perform these periodic flushes, based on the rate of flushing and how fast  redo  information is generated. See Also buffer pool, checkpoint, data files, flush, InnoDB, page, redo log.  An optimization for  InnoDB  tables that can speed up lookups using  =  and  IN  operators, by constructing a  hash index  in memory. MySQL monitors index searches for  InnoDB  tables, and if queries could benefit from a hash index, it builds one automatically for index  pages  that are frequently accessed. In a sense, the adaptive hash index configures MySQL at runtime to take advantage of ample main memory, coming closer to the architecture 6131