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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

530 MySQL Technical Reference for Version 4.0.3 column, then InnoDB executes SELECT MAX(auto-inc-column) FROM T, and assigns that value incremented by one to the column and the auto-increment counter of the table. We say that the auto-increment counter for table T has been initialised. InnoDB follows the same procedure in initializing the auto-increment counter for a freshly created table. Note that if the user species in an insert the value 0 to the auto-increment column, then InnoDB treats the row like the value would not have been specied. After the auto-increment counter has been initialised, if a user inserts a row where he explicitly species the column value, and the value is bigger than the current counter value, then the counter is set to the specied column value. If the user does not explicitly specify a value, then InnoDB increments the counter by one and assigns its new value to the column. The auto-increment mechanism, when assigning values from the counter, bypasses locking and transaction handling. Therefore you may also get gaps in the number sequence if you roll back transactions which have got numbers from the counter. The behaviour of auto-increment is not dened if a user gives a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specied integer type. 7.5.12 File Space Management and Disk I/O 7.5.12.1 Disk I/O In disk I/O InnoDB uses asynchronous I/O. On Windows NT it uses the native asynchronous I/O provided by the operating system. On Unix, InnoDB uses simulated asynchronous I/O built into InnoDB: InnoDB creates a number of I/O threads to take care of I/O operations, such as read-ahead. In a future version we will add support for simulated aio on Windows NT and native aio on those versions of Unix which have one. On Windows NT InnoDB uses non-buered I/O. That means that the disk pages InnoDB reads or writes are not buered in the operating system le cache. This saves some memory bandwidth. Starting from 3.23.41 InnoDB uses a novel le ush technique called doublewrite. It adds safety to crash recovery after an operating system crash or a power outage, and improves performance on most Unix avors by reducing the need for fsync operations. Doublewrite means that InnoDB before writing pages to a datale rst writes them to a contiguous tablespace area called the doublewrite buer. Only after the write and the ush to the doublewrite buer has completed, InnoDB writes the pages to their proper positions in the datale. If the operating system crashes in the middle of a page write, InnoDB will in recovery nd a good copy of the page from the doublewrite buer. Starting from 3.23.41 you can also use a raw disk partition as a datale, though this has not been tested yet. When you create a new datale you have to put the keyword newraw immediately after the data le-size in innodb_data_file_path. The partition must be >=

MySQL Reference Manual

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