1 - Oracle Server Architecture Overview

1. 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 1 - Oracle Server Architecture Overview • Background Processes • DBWR, LGWR, SMON, PMON, CKPT, ARCH • Other Processes • User processes • Server processes • PGA • Query Processing • Rollback Segments

2. 1 - Oracle Server Architecture Overview Oracle Instance Server Process SGA (System Global Area) PGA DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool (optional) User Process Large Pool (optional) DBWR LGWR SMON PMON CKPT RECO Other Files...Parameter, Password, Archive Logs ARCH LCKn Pnnn Dnnn SNPn Oracle Database Data files Redo Log Files Control Files

3. 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 Database Files

4. 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 Other Files...Parameter, Password, Archive Logs

5. Oracle Instance is comprised of the background processes and memory structures (SGA - Shared Global Area) The Oracle Instance Oracle Instance SGA (System Global Area) DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool Large Pool DBWR LGWR SMON PMON CKPT RECO ARCH LCKn Pnnn Dnnn SNPn

6. 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). Shared Pool SGA (System Global Area) DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool Large Pool

7. 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) Buffer Cache SGA (System Global Area) DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool Large Pool

8. Sizing the Buffer Cache DB_BLOCK_SIZE = 8192 8K 8K 8K DB_BLOCK_BUFFERS = 5 8K 8192 * 5 = 40960 Bytes (40K) 8K

9. 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

10. Optional areas of memory LARGE_POOL_SIZE Large Pool and Java Pool Oracle Instance SGA (System Global Area) DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool Large Pool DBWR LGWR SMON PMON CKPT RECO ARCH LCKn Pnnn Dnnn SNPn

11. 5 background processes are mandatory Database Writer (DBWR) Log Writer (LGWR) System Monitor (SMON) Process Monitor (PMON) Checkpoint (CKPT) Required as of Oracle 8i Background Processes DBWR LGWR SMON PMON CKPT RECO ARCH LCKn Pnnn Dnnn SNPn

12. 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

13. 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)

14. 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

15. 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

16. 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)

17. A user process always connects to the database via a server process Connecting to the Database Oracle Instance Server Process SGA (System Global Area) PGA DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool User Process Large Pool DBWR LGWR SMON PMON CKPT RECO ARCH LCKn Pnnn Dnnn SNPn

18. 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

19. Parse Execute Fetch Query Processing Oracle Instance Server Process SGA (System Global Area) PGA DatabaseBuffer Cache Shared Pool Redo Log Buffer Java Pool User Process Large Pool DBWR LGWR SMON PMON CKPT RECO ARCH LCKn Pnnn Dnnn SNPn

20. 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

21. Parse Execute Identifies the rows that are selected Fetch Returns the rows to the user process and ordered if necessary Query Processing

22. 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