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Disk and Page cache
chenzongzhi
Write path
Write Path
• The client sends a write command to the database (data is in
client's memory).
• The database receives the wr...
Numbers
Page cache
[chenzongzhi@mdb9921:~]$ sudo sysctl -a | grep dirty
[sudo] password for chenzongzhi:
vm.dirty_background_ratio...
Page Cache
• vm.dirty_background_ratio
Page cache
• vm.dirty_ratio
Example
Page Cache
• vm.dirty_writeback_centisecs
Page Cache
• vm.dirty_expire_centisecs
Situation I
• Decreasing the Cache
• vm.dirty_background_ratio = 5
• vm.dirty_ratio = 10
Situation II
• Increasing the Cache
• vm.dirty_background_ratio = 50
• vm.dirty_ratio = 80
Situation III
• Online Server
• vm.dirty_background_ratio = 5
• vm.dirty_ratio = 80
Why
Page Cache History
History
• Before 2.6.32: Linux used “Pdflush”
• After 2.6.32: Linux used Backing Device
Info(BDI) flush threads
• After 3.10...
Why
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Disk and page cache

  1. 1. Disk and Page cache chenzongzhi
  2. 2. Write path
  3. 3. Write Path • The client sends a write command to the database (data is in client's memory). • The database receives the write (data is in server's memory). • The database calls the system call that writes the data on disk (data is in the kernel's buffer). • The operating system transfers the write buffer to the disk controller (data is in the disk cache). • The disk controller actually writes the data into a physical media (a magnetic disk, a Nand chip, ...).
  4. 4. Numbers
  5. 5. Page cache [chenzongzhi@mdb9921:~]$ sudo sysctl -a | grep dirty [sudo] password for chenzongzhi: vm.dirty_background_ratio = 10 vm.dirty_background_bytes = 0 vm.dirty_ratio = 20 vm.dirty_bytes = 0 vm.dirty_writeback_centisecs = 500 vm.dirty_expire_centisecs = 3000
  6. 6. Page Cache • vm.dirty_background_ratio
  7. 7. Page cache • vm.dirty_ratio
  8. 8. Example
  9. 9. Page Cache • vm.dirty_writeback_centisecs
  10. 10. Page Cache • vm.dirty_expire_centisecs
  11. 11. Situation I • Decreasing the Cache • vm.dirty_background_ratio = 5 • vm.dirty_ratio = 10
  12. 12. Situation II • Increasing the Cache • vm.dirty_background_ratio = 50 • vm.dirty_ratio = 80
  13. 13. Situation III • Online Server • vm.dirty_background_ratio = 5 • vm.dirty_ratio = 80
  14. 14. Why
  15. 15. Page Cache History
  16. 16. History • Before 2.6.32: Linux used “Pdflush” • After 2.6.32: Linux used Backing Device Info(BDI) flush threads • After 3.10.0: Linux used kworker
  17. 17. Why

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