1 - Oracle Server Architecture Overview
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•
•
Oracle Database
- Data files, Control files,
Redo log files
Other Files
- Parameter file, Password
file, Archive logs
Oracle Instance
- SGA (Shared Global Area)
• Buffer Cache
• Shared Pool
» Library Cache
» Data Dictionary
Cache
• Redo Log Buffer
• Large Pool
• Java Pool
•
•
•
- Background Processes
• DBWR, LGWR, SMON,
PMON, CKPT, ARCH
Other Processes
- User processes
- Server processes
• PGA
Query Processing
Rollback Segments
1 - Oracle Server Architecture Overview
Oracle Instance
Server Process
SGA (System Global Area)
PGA
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
(optional)
User Process
Large Pool
(optional)
DBWR LGWR SMON PMON CKPT RECO
ARCH
Oracle
Database
LCKn
Pnnn
Dnnn
SNPn
Redo Log Files
Data files
Control Files
Other
Files...
Parameter,
Password,
Archive
Logs
Database Files
• Data files
- Store the data
- Can only be associated with 1 database
- Form logical units called a tablespace
• Redo log files
- Record all of the changes in the database
- Must have at least 2 groups
- Oracle recommends each group have 2 members on separate
devices
• Control files
- Database name
- Time stamp of database creation
- Name and locations of all data files and redo log files
Other Files
• Parameter files
- Store hundreds of parameters for the Oracle Instance
• Password file
- Authenticates which users are allowed to start and stop the
database
• Archive redo log files
- Copies of the redo log files that are saved so the database can
be recovered to a single point in time
Other
Files...
Parameter,
Password,
Archive
Logs
The Oracle Instance
• Oracle Instance is comprised of the background
processes and memory structures (SGA - Shared
Global Area)
Oracle Instance
SGA (System Global Area)
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
Large Pool
DBWR LGWR SMON PMON CKPT RECO
ARCH
LCKn
Pnnn
Dnnn
SNPn
Shared Pool
• Shared Pool
- Library Cache
• Stores the most recently used SQL statements
- Data Dictionary Cache (Row Cache)
• Stores definitions of objects contained in the database
• Used to validate object names and privileges
- Size is set by the SHARED_POOL_SIZE parameter in the
initialization file (parameter file).
SGA (System Global Area)
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
Large Pool
Buffer Cache
• Area of memory used to store the most recently used
data. Oracle knows which data is most likely to be
used again by maintaining a list of recently used blocks
called the LRU.
• Operation
- A request for data is made
- Oracle checks the buffer cache to see if it exists
- If not it grabs the block from disk (physical read)
SGA (System Global Area)
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
Large Pool
Sizing the Buffer Cache
DB_BLOCK_SIZE = 8192
8K
8K
8K
8K
8K
DB_BLOCK_BUFFERS = 5
8192 * 5 = 40960 Bytes (40K)
Redo Log Buffer
• Circular buffer that records all of the changes
occurring in the database
• Size
- Defined by LOG_BUFFER parameter
- Should be a multiple of DB_BLOCK_SIZE
- Usually small compared with total SGA size, small increase can
significantly enhance throughput
- As of Oracle 8 a 1 MB upper limit has been set on the
background write threshold which limits the negative impact of
a very large redo log buffer
• Data is written from the redo log buffer to the redo log
files by the log writer process
Large Pool and Java Pool
• Optional areas of memory
- LARGE_POOL_SIZE
Oracle Instance
SGA (System Global Area)
Database Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
Large Pool
DBWR LGWR SMON PMON CKPT RECO
ARCH
LCKn
Pnnn
Dnnn
SNPn
Background Processes
• 5 background processes are mandatory
-
Database Writer (DBWR)
Log Writer (LGWR)
System Monitor (SMON)
Process Monitor (PMON)
Checkpoint (CKPT)
• Required as of Oracle 8i
DBWR LGWR SMON PMON CKPT RECO
ARCH
LCKn
Pnnn
Dnnn
SNPn
Database Writer (DBWR)
• Writes modified (dirty) blocks in the buffer cache to the
data files
• Operates in batch mode
• When
- There are too many dirty blocks
• DB_BLOCK_MAX_DIRTY_TARGET
- A process scans a specified number of buffers in the LRU list
without finding a free buffer
- A time-out occurs (every 3 seconds)
- A checkpoint occurs
• You can configure additional DBWR processes by
setting the DB_WRITER_PROCESSES parameter in the
parameter file
Log Writer (LGWR)
• Writes information in the redo log buffer to the redo
logs
• Writes When:
-
Every time a commit is issued
When redo log buffer is 1/3 full
When there is more than 1 MB of changes
Before DBWR (DB Writer) completes cleaning modified buffer
blocks in the buffer cache by writing them to the data files
- When a timeout occurs (every 3 seconds)
System Monitor (SMON)
• Automatically recovers the instance during instance
failure
- Rolls forward transactions in the redo logs that have not been
written to disk
- Opens the database and makes locked data available
- Rolls back uncommitted transactions
Space Maintenance Functions:
• Coalesces free space
• De-allocates temporary segments
Process Monitor (PMON)
• Cleans up failed processes by handling existing
transactions and recovering memory
-
Rolls back existing transactions
Releases any locks held by the process
Releases other resources held by the process
Checks server and dispatcher processes and restarts them
when necessary
Checkpoint (CKPT)
• Used to synchronize data files, mandatory in Oracle 8i
- In Oracle 8 takes it handles the task of updating the data file
headers instead of log writer
- Frequent checkpoints increase the speed of database recovery
during instance failure but may hinder performance
• CHECKPOINT_PROCESS = True (Before 8i)
Connecting to the Database
• A user process always connects to the database via a
server process
Oracle Instance
Server Process
SGA (System Global Area)
PGA
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
User Process
Large Pool
DBWR LGWR SMON PMON CKPT RECO
ARCH
LCKn
Pnnn
Dnnn
SNPn
PGA (Program Global Area)
• PGA
- Used by only one process and is not shared
- Gets Created when Server Process Starts.
- Contents
• Stack space
• Session information
• Cursor state
• Sort area
Server Process
PGA
User Process
Query Processing
• Parse
• Execute
• Fetch
Oracle Instance
Server Process
SGA (System Global Area)
PGA
Database
Shared
Buffer
Pool
Cache
Redo
Log
Buffer
Java Pool
User Process
Large Pool
DBWR LGWR SMON PMON CKPT RECO
ARCH
LCKn
Pnnn
Dnnn
SNPn
Query Processing
• Parse
- SQL statement is passed from user process to server process
- Server process looks in shared pool to see if SQL already
exists
- Validates SQL by checking the syntax
- Checks the data dictionary to ensure all of the objects and
columns actually exist
- Locks on objects are obtained so the definition does not
change
- Checks the users privileges
- Determines the execution plan
- Loads the SQL and the plan into the shared SQL area
• Execute…continued
Query Processing
• Parse
• Execute
- Identifies the rows that are selected
• Fetch
- Returns the rows to the user process and ordered if necessary
Rollback Segments
• Record the “before” image before data is actually
changed in the database
• Purpose
- Allows user to roll back a transaction
- Allows other users to see the original image until the changes
have been committed
- Allows the database to be recovered if the instance fails