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Basics of SQL Transactions
“Big Picture” for understanding COMMIT
and ROLLBACK of SQL transactions
Files, Buffers, Listener, Service Threads,
and Transactions
Martti Laiho 2012-04-02
(Flat) SQL Transaction
[BEGIN TRANSACTION]
[SET TRANSACTION ISOLATION .. ]
SELECT …
INSERT …
UPDATE …
DELETE …
COMMIT or ROLLBACK
Database
Transaction log
Overview of a Database Server Instance
connections (sessions)
transactions
- SQL commands & results
network
main
memory
listener &
service threads
bufferpool(s)
DBMS instance
processes
(threads)
x
startup
log cache(s)
disc
DBMS
software
instance
control files
database
control files
tablespace
files
database files
transaction
logs
trace &
alert files
Database Instance Architectures
DB2 Instance
control files
directories
Oracle Instance
bufferpools
bufferpools
database
database
control files
directories
bufferpools
database
For every database the active
transaction logs build a circular chain
Oracle calls the chained active
transaction logs as redo-logs
SQL Server Instance
A server Instance may include
just one or multiple databases
depending on the DBMS product.
System
databases
master
bufferpool
database
database
In the following we will focus only
on the simple case of a single database
The active database transaction logs are allocated as
Virtual Log Files in a single file building a circular chain
Martti Laiho 2012-04-09
Files and Caches
Listener
Applications (clients)
listening for connection
requests of clients
...
Service threads
(agents)
Control Cache
Transaction control,
etc
Log Cache
Cache of before and
after images of rows
of active transactions
WAL
write-ahead-logging
protocol
(Bufferpool)
LRU protocol for freeing pages
Checkpoint
Data Files on disks
building the tablespace for tables
and indices
data pages
for rows
Circular chain of Transaction
Martti Laiho 2012-04-02
Data Cache
Cache of data pages
for fast data processing
minimizing disk I/O
Log Archive (for history)
Logs
Structures of Data Pages
Control data including pointers, dirty bit, LSN, etc
Page header
Page header
PCTFREE
percentage of
originally free
space on pages
Record of the row
row
Record
rest of the row
Slot directory
Address of a row is
RID (ROWID)
File# : Page# : Slot#
Every data file has an internal file# in the database
and the file space is managed as a sequence of pages.
Page# is the ordinal number of the page in the file.
A typical page size nowadays is 4, 8, 16 or 32 KB.
Martti Laiho 2012-04-06
Slot directory
A row may continue on other pages
or original record may contain only
a link to the new address, preserving
the original RID value of the row’s
Address.
SQL Session: A client starts its SQL connection ..
connect?
Listener
...
Control Cache
Service threads
(agents)
Data Cache
(Bufferpool)
Log Cache
Data Files
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
.. gets a service thread
connect?
Listener
...
Control Cache
Service threads
(agents)
Data Cache
(Bufferpool)
Log Cache
Data Files
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
.. Client enters a SQL command to update a row (still on disk)
This first SQL command
starts a new transaction
update T set .. where id=..
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
Data Cache
(Bufferpool)
Log Cache
# begin
Data Files
The transaction gets
transaction ID (#)
and a begin_transaction
record is written to the
Log Cache
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
DBMS reads the page of the row to bufferpool
update T set .. where id=..
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
...
Data Cache
(Bufferpool)
Log Cache
# begin
Data Files
. ...
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
The row is updated and the updated page is marked with the Dirty Bit
update T set .. where id=..
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
xxx
dirty bit
Data Cache
(Bufferpool)
Log Cache
# begin
Data Files
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
A log record of the before and after image of the row is written to log cache
update T set .. where id=..
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
xxx
dirty bit
Data Cache
(Bufferpool)
Log Cache
# begin
xxx
#
#tr & before image & after image
Data Files
Log Archive
Transaction Logs
Martti Laiho 2012-04-02
On commit the log records of the transaction are written to the transaction log
COMMIT
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
xxx
dirty bit
Data Cache
(Bufferpool)
Log Cache
# begin
xxx
#
Data Files
Log Archive
# begin #
xxx
# commit
Transaction Logs
Martti Laiho 2012-04-02
.. but in case of ROLLBACK the before images are returned to original addresses
ROLLBACK
Listener
...
Control Cache
Service threads
(agents)
Transaction #tr
dirty bit
Data Cache
(Bufferpool)
Log Cache
# begin
xxx
#
Data Files
Log Archive
# begin #
xxx
# rollback
Transaction Logs
Martti Laiho 2012-04-03
.. Later the client closes its SQL connection
disconnect
Listener
...
Control Cache
xxx
dirty bit
Service threads
(agents)
Data Cache
(Bufferpool)
Log Cache
Data Files
Log Archive
# begin #
xxx
# commit
Transaction Logs
Martti Laiho 2012-04-02
.. Some other pages may also get updated by SQL sessions of clients
Listener
...
Service threads
(agents)
Control Cache
#tr(s)
...
xxx
X
xx
Data Cache
(Bufferpool)
Log Cache
#
#
X xx
Data Files
Log Archive
# begin #
xxx
# commit
Transaction Logs
Martti Laiho 2012-04-02
…
From time to time a checkpoint stops the services ..
Listener
...
Control Cache
#tr(s)
Service threads
(agents)
Checkpoint
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
#
#
X xx
write-ahead-logging (WAL)
Data Files
1.
# begin #
Log Archive
xxx
# commit
Transaction Logs
Martti Laiho 2012-04-07
…
and all dirty pages are written to data files clearing the dirty bits
Listener
...
Control Cache
#tr(s)
Service threads
(agents)
Checkpoint
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
2.
Data Files
xxx
xxx
Log Archive
xxx
# begin #
#
#
X
# commit
xx
Transaction Logs
Martti Laiho 2012-04-07
…
and writes checkpoint record
Listener
...
Control Cache
#tr(s)
Service threads
(agents)
Checkpoint
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
Data Files
Log Archive
Checkpoint record
- Chk Begin CBg ### CEn
- #tr list
- Chk End
3.
xxx
# begin #
#
#
X
xx
# commit
CBg ### CEn
Transaction Logs
Martti Laiho 2012-04-08
…
After the checkpoint the serving of clients can continue
Listener
...
Service threads
(agents)
Control Cache
#tr(s)
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
Data Files
xxx
xxx
Log Archive
xxx
# begin #
#
#
X
xx
# commit
CBg ### CEn
Transaction Logs
Martti Laiho 2012-04-02
…
Contents of full transaction log files are copied to archive ..
Listener
...
Service threads
(agents)
Control Cache
#tr(s)
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
Data Files
xxx
xxx
Log Archive
xxx
# begin #
#
#
X
xx
# commit
CBg ### CEn
Transaction Logs
Martti Laiho 2012-04-02
…
# begin #
xxx
# commit
…
.. and the space of the copied transaction log file can be reused
Listener
...
Service threads
(agents)
Control Cache
#tr(s)
...
xxx
xxx
Data Cache
(Bufferpool)
Log Cache
Data Files
xxx
xxx
Log Archive
#
#
X
xx
CBg ### CEn
Transaction Logs
Martti Laiho 2012-04-02
# begin #
xxx
# commit
…
A logging test using SQL Server 2012
based on the original exercise by Kari Silpiö
21
Page header
10
11
10 22
11
21
2123
Slots
Page ID 1:77
..3210
Some Notes ..
•
•
•
•
•
The Least Recently Used (LRU) procedure is needed when DBMS needs to
recover free space in the bufferpool(s). The pages which have been unused
for the longest time and have dirty bit reset, will be cleared for new pages to
be read from the data files.
The active, circular transaction log files are the most important files of a
database, since they contain data of the latest committed transactions!
Dual logging (replicated) is recommended for these, and on dedicated disks.
Based on the last checkpoint record and the circular transaction log a DBMS
can recover the database after a soft crash automatically by roll-back recovery
of failed and roll-forward recovery of the committed transactions to the level
of the latest committed transasction before the soft crash.
In case of hardware crash the database content need to be restored from the
database backup and the applying roll-forward recovery of the transaction
history from the archive and the latest circular logs, and finally rollback
recovery of the latest failed transactions.
These topics are covered in separate tutorials.