MySQL 45 lecture learning notes (12) MySQL will "shake" for a while

One . scene

      A modification SQL sentence , It's very fast in normal times , But sometimes it's very slow , The scene is difficult to reproduce , Short duration .

The reasons causing ：

• InnoDB When processing update statements , Only do the disk operation of writing log . This log is called redo log（ Redo log ）
• After updating the memory redo log after , Return to the client , This update is successful

When the memory is full , When you have to synchronize to memory , It will be slower

   flush Namely Data in memory is written to disk

• Dirty page ： When memory data page and disk data page atypism When
• Clean pages ： When memory And the data page on the disk The content is consistent

Two . Generate the scene of cleaning dirty pages

• redo log Full of , The system should stop all update operations , take checkpoint Push forward ,redo log Make room to continue writing .
• There is not enough memory in the system , A new memory page is required , Data pages need to be eliminated when memory is insufficient , Write dirty pages to disk
• MySQL Think of the system as “ Free ” When the
• MySQL Normally closed

Analysis of the first two situations ：

• redo log It's full. , To brush flush Dirty page , This situation InnoDB Try to avoid , The system no longer accepts updates , All updates must be blocked
• There's not enough memory , Write dirty pages to disk , This situation is normal

InnoDB Use cache pool （buffer pool） Manage memory , Memory pages in the cache pool have three states ：

• not used
• Used are all clean pages
• Used but dirty pages

InnoDB Try to use memory when implementing strategies , For a long-running Library , Not used There are few pages

Brush dirty pages yes Normal behavior , But the following will significantly affect performance ：

1. There are too many dirty pages to be eliminated , This leads to significantly longer query response time
2. The log is full of , All updates are blocked , Writing performance has dropped to 0

3、 ... and .InnoDB Control strategy of brushing dirty pages  

InnoDB Of the host IO Ability , such InnoDB To know when you need to brush dirty pages with all your strength , How fast can I brush .

    innodb_io_capacity This parameter

• It is recommended to set the value to disk IOPS
• On disk IOPS It can be used fio Tool testing