MySQL is dirty

MySQL Brushwork

Generation of dirty pages

• When updating data , Only the data pages in memory are updated , No disk updates
• The data page in memory is inconsistent with the data page in disk , It's called dirty pages

Brushwork

• Save data pages in memory to disk
• meanwhile , Delete... Related to this page redo log, advance check point

Why brush dirty ？

• There are too many dirty pages in memory , Out of memory ( There will be performance problems )
• redo log The file is full , Need to advance check point( There will be performance problems )
• The system is idle , Brush dirty in advance , Prevent the above situation
• MySQL Before closing , Save the data

Avoid being forced to brush dirty

• Correctly inform InnoDB, Hard disk performance of the server
• Configure a reasonable upper limit of dirty pages
• control " Brush the dirt by the way " Strategy

The server IO To configure

• Configuration item ：innodb_io_capacity
• It is used to inform the server of the hard disk performance

fio -filename=iotest_file -direct=1 -iodepth 1 -thread -rw=randrw -ioengine=psync -bs=16k -size=500M -numjobs=10 -runtime=10 -group_reporting -name=mytest

• common IOPS：(I/O per second)
  • 7200 rpm SATA,70IOPS
  • 10000 rpm FC,125IOPS
  • SSD SATA,3000~4000IOPS

Configure a reasonable upper limit of dirty pages

• Configuration item ：innodb_max_dirty_pages_pct： The ratio of the number of pages in the disk to the number of dirty pages
• When the dirty page ratio approaches this value , It will speed up the cleaning
• It is recommended to keep the default value 75

control " Brush the dirt by the way " Strategy

• Configuration item ：innodb_flush_neighbors
• Traditional disks have the best continuous read and write performance , Try to brush consecutive pages
• SSD It is recommended to set 0(8.0 It has been acquiesced that 0)