Introduction to Oracle
Physical Structure
 Logical Structure
 SGA / PGA
 Background Processes
Backup Methods
Computer Science Database – CS01
 Administrative Tasks
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Physical Structures
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Datafiles (*.dbf)
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Control Files (*.ctl)
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The datafiles contain all the database data. The data of logical database
structures, such as tables and indexes, is physically stored in the datafiles
allocated for a database.
Every Oracle database has a control file. A control file contains entries that
specify the physical structure of the database such as Database name and the
Names and locations of datafiles and redo log files.
Redo Log Files (*.log)
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The primary function of the redo log is to record all changes made to data. If a
failure prevents modified data from being permanently written to the datafiles,
then the changes can be obtained from the redo log, so work is never lost.
Physical Structures (cont’d)
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Archive Log Files (*.log)
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Parameter Files (initSID.ora)
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Oracle automatically archives log files when the database is in ARCHIVELOG
mode. This prevents oracle from overwriting the redo log files before they have
been safely archived to another location.
Parameter files contain a list of configuration parameters for that instance and
database.
Alert and Trace Log Files (*.trc)
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Each server and background process can write to an associated trace file. When an internal
error is detected by a process, it dumps information about the error to its trace file. The
alert log of a database is a chronological log of messages and errors.
Logical Structures
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Tablespaces
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A database is divided into logical storage units called tablespaces, which group
related logical structures together. One or more datafiles are explicitly created for
each tablespace to physically store the data of all logical structures in a
tablespace.
Oracle Data Blocks
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At the finest level of granularity, Oracle database data is stored in data blocks.
One data block corresponds to a specific number of bytes of physical database
space on disk. The standard block size is specified by the DB_BLOCK_SIZE
initialization parameter.
Logical Structures (cont’d)
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Extents
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The next level of logical database space is an extent. An extent is a specific
number of contiguous data blocks, obtained in a single allocation, used to store a
specific type of information.
Segments
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Above extents, the level of logical database storage is a segment. A segment is a
set of extents allocated for a certain logical structure. The different types of
segments are :
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Data segment – stores table data
Index segment – stores index data
Temporary segment – temporary space used during SQL execution
Rollback Segment – stores undo information
Logical Structures (cont’d)
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Schema Overview
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A schema is a collection of database objects. A schema is owned by a database
user and has the same name as that user. Schema objects are the logical
structures that directly refer to the database's data. Schema objects include
structures like tables, views, and indexes.
Oracle Instance
An Oracle database server consists of an
Oracle database and an Oracle instance.
Every time a database is started, a system
global area (SGA) is allocated and Oracle
background processes are started. The
combination of the background processes
and memory buffers is called an Oracle
instance.
System Global Area (SGA)
The System Global Area (SGA) is a shared memory region that contains data and
control information for one Oracle instance. Users currently connected to an Oracle
database share the data in the SGA. The SGA contains the following memory
structures :
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Database Buffer Cache
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Database buffers store the most recently used blocks of data. The set of
database buffers in an instance is the database buffer cache. The buffer cache
contains modified as well as unmodified blocks. Because the most recently (and
often, the most frequently) used data is kept in memory, less disk I/O is
necessary, and performance is improved.
System Global Area (cont’d)
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Redo Log Buffer of the SGA
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The redo log buffer stores redo entries—a log of changes made to the database.
The redo entries stored in the redo log buffers are written to an online redo log,
which is used if database recovery is necessary. The size of the redo log is static.
Shared Pool of the SGA
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The shared pool contains shared memory constructs, such as shared SQL areas.
A shared SQL area is required to process every unique SQL statement submitted
to a database. A shared SQL area contains information such as the parse tree
and execution plan for the corresponding statement.
Program Global Area (PGA)
PGA is a memory buffer that contains data and control information for a
server process. A server process is a process that services a client’s
requests. A PGA is created by oracle when a server process is started. The
information in a PGA depends on the oracle configuration. The PGA area is
a non-shared area of memory created by oracle when a server process is
started.
The basic difference between SGA and PGA is that PGA cannot be shared
between multiple processes in the sense that it is used only for
requirements of a particular process whereas the SGA is used for the whole
instance and it is shared.
Oracle Background Processes
An Oracle database uses memory structures and processes to manage and access the
database. All memory structures exist in the main memory of the computers that
constitute the database system. Processes are jobs that work in the memory of these
computers.
Oracle creates a set of background processes for each instance. The background
processes consolidate functions that would otherwise be handled by multiple Oracle
programs running for each user process. They asynchronously perform I/O and
monitor other Oracle processes to provide increased parallelism for better
performance and reliability.
The most common background processes are :
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System Monitor – SMON
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This database background process performs instance recovery at the start of the
database. SMON also cleans up temporary segments that are no longer in use
and recovers dead transactions skipped during crash and instance recovery
because of file-read or offline errors. It coalesces i.e. combines contiguous free
extents into larger free extents.
Background Processes (cont’d)
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Process Monitor - PMON
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This database background process cleans up failed user processes. PMON is
responsible for releasing the lock i.e. cleaning up the cache and freeing resources
that the process was using. Its effect can be seen when a process holding a lock
is killed.
Database Writer - DBWR
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This background process is responsible for managing the contents of the data
block buffer cache and dictionary cache. DBWR performs batch writes of
changed block. Since Oracle uses write-ahead logging, DBWR does not need to
write blocks when a transaction commits. In the most common case, DBWR
writes only when more data needs to be read into the system global area and
too few database buffers are free. The least recently used data is written to the
datafiles first.
Although there is only one SMON and one PMON process running per database
instance, one can have multiple DBWR processes running at the same time. Note
the number of DBWR processes running is set via the DB_WRITER_PROCESSES.
Background Processes (cont’d)
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Log Writer - LGWR
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This background process manages the writing of the contents of the redo log
buffer to the online redo log files. LGWR writes the log entries in batch form. The
Redo log buffers entries always contain the most up-to-date status of the
database.
Archiver - ARCH
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The Archiver process reads the redo log files once Oracle has filled them and
writes a copy of the used redo log files to the specified archive log
destination(s). Actually, for most databases, ARCH has no effect on the overall
system performance. On some large database sites, however, archiving can have
an impact on system performance.
Background Processes (cont’d)
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Checkpoint - CKPT
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Recover - RECO
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All modified information in database buffer in the SGA is written to the datafiles
by a database write process (DBWR). This event indicates a checkpoint. The
checkpoint process is responsible for signaling DBWR at checkpoints and
updating all of the datafiles and control files of the database.
The recover process automatically cleans up failed or suspended distributed
transactions.
Job Queue Processes
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Job queue processes are used for batch processing. They run user jobs. They
can be viewed as a scheduler service that can be used to schedule jobs as
PL/SQL statements or procedures on an Oracle instance. Given a start date and
an interval, the job queue processes try to run the job at the next occurrence of
the interval.
Computer Science Database
Server Information
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Sun e4500
8GB Ram
8 x 400mhz CPU
32GB Disk for
Oracle
4mm DAT DDS3
Tape Backup
Computer Science Database
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Instance Name : CS01 (v$database)
Instance Version : 8.1.6.0.0
Tablespaces : (dba_tablespaces)
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SYSTEM – holds all system tables
INDEX01 – user indexes
USERS01 – user tables
USERS02 – user tables (faculty)
RBS – rollback segments
Backup Methods
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Cold Backup (aka Consistent Backups)
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The only way to make a consistent whole database
backup is to shut down the database with the
NORMAL, IMMEDIATE, or TRANSACTIONAL options
and make the backup while the database is closed.
Advantage : No recovery is required after datafiles
are restored – quicker restore
Disadvantage : No access to database during backup
time (depends on size/system speed)
Backup Methods (cont’d)
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Hot Backup (aka Inconsistent Backups)
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If the database must be up and running 24 hours a
day, seven days a week, then you have no choice but
to perform inconsistent backups of the whole
database. A backup of online datafiles is called an
online backup. This requires that you run your
database in ARCHIVELOG mode.
Advantage : Database remains open during backup
Disadvantage : Large databases may have
performance impact during backup, recovery takes
longer and is slightly more complex
Backup Methods (cont’d)
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Logical backup (Export)
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Logical backups are exports of schema objects, like
tables and stored procedures, into a binary file.
Oracle utilities are used to move Oracle schema
objects in and out of Oracle.
Not recommended for backup of a whole database,
but useful for backing up individual objects or
schemas or moving data into another database
Administrative Tasks
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Daily Checks
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Check database availability
Check logs / trace files
Check free space / resources
Check for invalid objects
Check for broken jobs
Verify backup
Administrative Tasks (cont’d)
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Weekly Tasks
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Collect statistics (database job)
Archive / delete log files
Run performance reports (statspack)
Administrative Tasks (cont’d)
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Others
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Applying patches
Database upgrades
New Database installations
Creating user accounts
More Information
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Oracle 10g Release 2 Database Documentation
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Oracle Database / SQL Help
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http://www.oracle.com/pls/db102/
http://asktom.oracle.com/
http://www.oracle.com/technology//index.html
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