How MySQL works

1. MySQL stuff

2. About me
• Before REA I worked for a hosting company for 4
years
• Learnt a lot about MySQL during this period
• I started as an OPS engineer at REA and did some
database stuff here
• I havent done much MySQL for two years
!
• In this period we’ve had
• MySQL 5.6 come out
• RDS MySQL improvements
!
!

3. Today
• High level overview of MySQL
• Look inside MySQL server
• Talk through how InnoDB performs queries
• Schema changes and online schema changes
!
!

4. Background
• Second biggest relational database in the world
• Anyone know what the biggest is?
!
• Used by many of the big web shops
• Facebook for all feed data
• Etsy for user & data
• Twitter for tweet persistence
• …
!
• Sun bought MySQL AB in 2008, people freaked out
• Oracle bought Sun in 2010, people freaked out
• Big improvements over the past few years (5.5 / 5.6)
!

5. How it works
mysqld
mysql client tcp:3306
filesocket
ServerClient
libmysql
mysql lib
ORM
host$ mysql
host$ mysqld_safe!
host$ mysqld

6. Big features
• Pluggable storage engines is massive flexibility
• You can use a different storage engine per table
!
!
• Common storage engines
• InnoDB - ACID compliant store
• MyISAM - one of the original storage engines -
avoid
• Lots and lots of others
!

7. Inside mysqld
mysqld
storage engine(s)
MyISAM / InnoDB / Memory / etc
tcp:3306
filesocket
Getting the query
and responding
bits and
bytes stuff
API

8. Inside mysqld
mysqld
storage engine(s)
MyISAM / InnoDB / Memory / etc
tcp:3306
filesocket
API

9. Inside mysqld
mysqld
storage engine(s)
MyISAM / InnoDB / Memory / etc
tcp:3306
filesocket
Parse query and
convert to SE
API calls.
API

10. Inside mysqld
mysqld
Storage engines(s)
tcp:3306
filesocket
API
query cache
binary log
join / sort buffers
Binary Log for replication
Buffers: Kinda how much
batching to storage API
to save cost

11. The bad parts - binary log
mysqld
tcp:3306
filesocket
query cache
binary log
join / sort buffers
Because you can mix and
match storage engines,
replicating state has to be
done at the lowest
common denominator
Innodb
Table 1
MyISAM
Table 2

12. The bad parts - binary log
mysqld
Innodb
Table 1
tcp:3306
filesocket
query cache
binary log
join / sort buffers
Highly concurrent InnoDB
needs its queries,
serialized down
to replicate to slave
MyISAM
Table 2

13. Compare to other DBs
DB server
tcp:x
filesocket?
performance
knobs
transaction
log

14. The bad parts - query cache
mysqld
Innodb
Table 1
tcp:3306
filesocket
query cache
binary log
join / sort buffers
Can only work on an
entire table level &
uses LRU
MyISAM
Table 2

15. The bad parts - query cache
mysqld
Innodb
Table 1
tcp:3306
filesocket
query cache
binary log
join / sort buffers
All ‘update/insert/delete’
blocks all transactions
to ensure correct state.
It actually slows !
things down
MyISAM
Table 2

16. Inside mysqld
mysqld
storage engine(s)
MyISAM / InnoDB / Memory / etc
tcp:3306
filesocket
API
Where 10%* of !
the work is done
Where 90% !
of the work !
is done
*Finger in the air calculation

17. Summary
• Turn off query cache! It is hurting you!
• Binary log is a necessary evil for replication
• Dont play with the performance knobs (sort buffer /
join buffer / etc)
• Turning the 10% - Focus on the 90%
• Use InnoDB for all the tables (unless they’re system
tables)
!
!
• So…lets look at the 90%
!
!

18. InnoDB
mysqld
InnoDB
tcp:3306
filesocket
API

19. Unknown gems
mysqld
InnoDB
tcp:3306
filesocket
API
handlersocket/
memcache
Innodb

20. Unknown gems
mysqld
InnoDB
tcp:3306
filesocket
API
handlersocket/
memcache
Innodb
Memcache API
that talks directly
to InnoDB
!
Removes the SQL
layer and is just K/V
!
Google: handlersocket
memcache InnoDB

21. Key parts of InnoDB
Innodb
API

22. Key parts of InnoDB
Table space
(whats on disk)
redo log
Innodb
bufferpool
memory!
ib_log1!
ib_log2!
sometable.ibd!
ib_data!

23. Key parts of InnoDB
Table space
redo log
bufferpool
• Each block is a page
• Just think of it as a row with some extra stuff like
version number
• InnoDB is ACID meaning each connection has to
have its own view of the world!
• Extra metadata at the top about where everything is

24. Key parts of InnoDB
Table space
redo log
bufferpool
• Black is the base metadata

25. What is bufferpool
Table space
redo log
bufferpool
• In memory version of hot tablespace pages

26. What is redo log
Table space
redo log
bufferpool
• An append only log file of changes

27. What is table space
Table space
redo log
bufferpool
• The raw files - but it may not be always up-to-date

28. Lets run some queries
Table space
redo log
bufferpool
• First - lets fill up our database with data

29. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;

30. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
mysqld parses the query and sends !
API calls to InnoDB

31. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Is red in bufferpool?

32. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Load red into buffer pool
perform random !
read disk activity!
At this point, the time to respond!
is dependent on disk speed!

33. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Respond to API calls to mysqld

34. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Parse results !
second pass sort if necessary !
return to client

35. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red; #again

36. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
mysqld parses the query and sends !
API calls to InnoDB

37. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Is red in bufferpool? - yes

38. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Respond to API calls to mysqld

39. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from red;
Parse results !
second pass sort if necessary !
return to client

40. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from brown;
What if bufferpool is full?

41. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from blue;
Assume metadata knows page requirements!
LRU on the bufferpool to find space and evict

42. Lets run some queries
Table space
redo log
bufferpool
• mysql> select * from blue;
!
Load in blue as before and return

43. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
Is page in bufferpool? yes

44. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
Update the page in bufferpool and increment !
page version number

45. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
Write the new page to the redo log!
what tablespace is up to!

46. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
Update metadata telling it that this is the !
new state of the page!
perform sequential !
write disk activity to redo log!
This is pretty fast!
what tablespace is up to!

47. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
Send the OK back to mysqld!
what tablespace is up to!

48. Lets run some queries
Table space
redo log
bufferpool
• mysql> update blue set A=B where ID=1;
!
At sometime in the near future, do a semi sequential !
parse changes and update the tablespace
what tablespace is up to!
semi sequential write!
Not that bad !
!

49. Lets run some queries
Table space
redo log
bufferpool
• mysql> insert into yellow values
(1,2,3,4);
!
Is space in bufferpool?!

50. Lets run some queries
Table space
redo log
bufferpool
• mysql> insert into yellow values
(1,2,3,4);
!
Is adjacent pages in bufferpool?!
Random read !
(but not too big)

51. Lets run some queries
Table space
redo log
bufferpool
• mysql> insert into yellow values
(1,2,3,4);
!
Send OK back to client!

52. Key take aways
Table space
redo log
bufferpool
• Cold databases are slow!
• bufferpool is empty and everything requires random disk
reads
• The bigger the buffer pool you have, the quicker you will be!
(NUMBER 1 factor on performance!)!
• Redo log is the second slowest part
• we can tune this
innodb_flush_logs_at_trx_commit=1
innodb_buffer_pool_size= ~80% memory!

53. Schema changes
• Everyone hates them
• They are not transactional
• They are blocking (well most)
• How do they work?
!

54. Remember this?
mysqld
Storage engines(s)
tcp:3306
filesocket
API
query cache
binary log
join / sort buffers

55. Lets add a few more
Storage engines(s)
tcp:3306
filesocket
API
table meta data
binary log
Table triggers
functions / procedures

56. mysqld
tcp:3306
filesocket
mytbl
Schema changes
mytbl meta

57. mytbl
mysql> CREATE TABLE mytbl
(a INT, b CHAR (20), INDEX (a)) ENGINE=InnoDB;
Schema changes

58. mytbl
Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;

59. mytbl
Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;
!
Wait for any open transactions to close and then !
lock access to the table!
block

60. mytbl
Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;
!
Create a new ‘hidden’ table with the new schema!
block
.mytbl

61. mytbl
Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;
!
Do an internal ‘mysqldump’ / ‘mysqlrestore’!
block
.mytbl

62. mytbl
Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;
!
Do a switch-a-roo!
block
.mytbl

63. Schema changes
mysql> ALTER TABLE mytbl
ADD COLUMN age INT;
!
Unblock and send OK back to client!
mytbl

64. Online schema changes
• Some changes can be done without ‘locking’
• Column additions
• Index additions
!
• Any removal cannot be done online
• Unless you do it yourself
!

65. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta

66. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER…
pt-online-schema-change
soundclouds lhm
etc

67. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
!
Creates another
table with the
updated
schema in the
client!

68. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
!
Creates a set of
triggers on the
main table!
triggers

69. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
!
selects the
entire table (in
batches) and
inserts into the
new table!
!
!
This can take a
long time!
triggers

70. mysqld
mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
triggers
client
mysql> select
!
Whilst the new
table is being
built, selects
still go to the
old table!

71. mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
triggers
client
mysql> UPDATE
!
Updates cause
the trigger to
fire which
updates the
new table being
built. (Inserts
the row if its
still hasn't been
copied)!
!
Deletes happen
the same way!

72. mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
triggers
client
mysql> INSERT
!
Inserts also
happen the
same way

73. mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
triggers
!
Eventually the
table
population
finishes and a
‘RENAME table’
action is
performed
(which is quick)

74. mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… .mytbl meta
.mytbl
triggers
!
Eventually the
table
population
finishes. This is
the only
‘blocking’ time
block block

75. mytbl
Online schema changes 5.1/5.5
mytbl meta
client
mysql> ALTER… !
The schema
change tool
finishes

76. Online schema changes in 5.6
• All that ‘switch-a-roo’ logic is now within mysqld itself
!

77. Summary about schema changes
• You ‘double your IO’ when you’re online schema
changes!
• Binary log is outside of this - so watch out for your
slaves table locking if you don't use the right flags
!
!