1 - Oracle Server Architecture Overview

• 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

User Process

Server Process

Other Files...Parameter,Password,Archive Logs

PGA

Oracle Database Data files

Redo Log Files

Control Files

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

DBWR LGWR SMON PMON CKPT RECO

ARCH LCKn Pnnn Dnnn SNPn

Oracle Instance

Java Pool(optional)

Large Pool(optional)

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)

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

DBWR LGWR SMON PMON CKPT RECO

ARCH LCKn Pnnn Dnnn SNPn

Oracle Instance

Java Pool

Large Pool

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).

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

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)

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

Java Pool

Large Pool

Sizing the Buffer Cache

8K

8K

8K

8K

8K

DB_BLOCK_SIZE = 8192

DB_BLOCK_BUFFERS = 5

8192 * 5 = 40960 Bytes (40K)

• 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

Redo Log Buffer

Large Pool and Java Pool

• Optional areas of memory- LARGE_POOL_SIZE

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

DBWR LGWR SMON PMON CKPT RECO

ARCH LCKn Pnnn Dnnn SNPn

Oracle Instance

Java Pool

Large Pool

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

• 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

Database Writer (DBWR)

• 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)

Log Writer (LGWR)

• 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

System Monitor (SMON)

• 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

Process Monitor (PMON)

• 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)

Checkpoint (CKPT)

Connecting to the Database

• A user process always connects to the database via a server process

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

DBWR LGWR SMON PMON CKPT RECO

ARCH LCKn Pnnn Dnnn SNPn

Oracle Instance

User Process

Server Process

PGA

Java Pool

Large Pool

• 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

PGA (Program Global Area)

User Process

Server Process

PGA

Query Processing

• Parse• Execute• Fetch

DatabaseBufferCache

Shared Pool

RedoLog

Buffer

SGA (System Global Area)

DBWR LGWR SMON PMON CKPT RECO

ARCH LCKn Pnnn Dnnn SNPn

Oracle Instance

User Process

Server Process

PGA

Java Pool

Large Pool

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

• 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

Rollback Segments