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An illustrated guide to the buffer cache
Selena DeckelmannBased on Greg Smith’s “Inside the Buffer Cache”
http://www.westnet.com/∼gsmith/content/postgresql
Databases
Table
Table
Table
Databases are made of tables...
Table$PGDATA/base/
A table is just a directory...
Table$PGDATA/base/
Table
Table
You can have more than one table.
Table$PGDATA/base/
Table
TableFile
File
File
File
... Up to 1 GB
8K8K8K8K8K8K8K...
Made of Made of
Files are made up of 8K blocks.*
* you can change that at compile time.
shared_buffers
shared_buffers
ptr ptr ptr ptr ptr ...
An array of pointers to 8K blocks(and some other stuff)
shared_buffers is configured at database start.You have to restart Postgres to change it.
shared_buffers:the other stuff
FREE FREE FREE FREE FREE FREE
In the beginning...
shared_buffers:the other stuff
FREE FREE FREE FREE FREE FREE456 12 100 612 32 409
shared_buffers:the other stuff
X X X X X X456 12 100 612 32 409
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409Tag
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409FLAGS FLAGS FLAGS FLAGS FLAGS FLAGS
Tag
Status
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED flags flags flags flags flags
Tag
Status
UPDATE mytable (status) VALUES(‘my cat is the best ever’)where user = ‘selenamarie’;
{ }
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY flags flags flags flags
Tag
Status
COMMIT;{ }
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
Tag
Status
BLAH BLAH BLAH;{ }
shared_buffers:the other stuff
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
Tag
StatusUsage
shared_buffers:the lifecycle
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
myPgProcess calls:BufferAlloc(‘base/file1’, 100);{ }
shared_buffers:the lifecycle
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY PINNED OTHER OTHER OTHER
1 5 5 4 3 0
myPgProcess calls:BufferAlloc(‘base/file1’, 100);{ }
shared_buffers:the lifecycle
myPgProcess calls:BufferAlloc(‘base/file1’, 101);{ }
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
shared_buffers:the lifecycle
myPgProcess calls:BufferAlloc(‘base/file1’, 101);{ }
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
Eviction!
Who’s it gonna be?
shared_buffers:the lifecycle
myPgProcess calls:BufferAlloc(‘base/file1’, 101);{ }
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
0 5 5 4 3 0
Who’s it gonna be?
Scan the cache!Usage != 0,
so decrement.
shared_buffers:the lifecycle
myPgProcess calls:BufferAlloc(‘base/file1’, 101);{ }
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409PINNED DIRTY OTHER OTHER OTHER OTHER
0 4 4 3 2 0
Who’s it gonna be?
Usage == 0,Goodbye!
shared_buffers:the lifecycle
myPgProcess calls:BufferAlloc(‘base/file1’, 101);{ }
X X X X X Xbase/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 101PINNED DIRTY OTHER OTHER OTHER PINNED
0 4 4 3 2 1
Who’s it gonna be?
shared_buffers:optimizations*
SELECT pg_relation_size(‘mytable’);VACUUM mytable;{ }
X X X X X X X ...base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
Result is greater than:
shared_buffers/4
* Version 8.3 and later
shared_buffers:optimizations
SELECT pg_relation_size(‘mytable’);VACUUM mytable;{ }
X X X X X X X ...base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
256K ringbuffer allocated!
* Version 8.3 and later
shared_buffers:optimizations
SELECT pg_relation_size(‘mytable’);VACUUM mytable;{ }
X X X X X X X ...base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
256K ringbuffer allocated!
OVERSIMPLIFICATION WARNING!
Safe duringVACUUM
* Version 8.3 and later
