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下边这段话主要描写了innodb如何来保障可恢复,和减少fsync的数量的

原创 Linux操作系统 作者:bulletming 时间:2019-02-12 11:45:04 0 删除 编辑

12.5.14.1. InnoDB Disk I/O

InnoDB uses simulated asynchronous disk I/O: InnoDB creates a number of threads to take care of I/O operations, such as read-ahead.

There are two read-ahead heuristics(启发式) in InnoDB:

  • In sequential read-ahead, if InnoDB notices that the access pattern to a segment in the tablespace is sequential, it posts in advance a batch of reads of database pages to the I/O system. (innodb注意到大批量的顺序读相应,会事先去批量读数据)

  • In random read-ahead, if InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool, it posts the remaining reads to the I/O system. (不懂)

InnoDB uses a novel file flush technique called doublewrite. It adds safety to recovery following an operating system crash or a power outage, and improves performance on most varieties of Unix by reducing the need for fsync() operations.

Doublewrite means that before writing pages to a data file, InnoDB first writes them to a contiguous tablespace area(比较觉得这个就是log_buffer,记录到连续的空间意味着将原本的随机写先通过顺序写保存起来) called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed does InnoDB write the pages to their proper positions in the data file(写数据前要先写doublewrite buffer). If the operating system crashes in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during recovery.

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