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The Basics of Oracle Architecture
By: Budi Raharjo
http://mbraharjo.blogspot.com
As an Oracle DBA, you must be understand the concepts of Oracle architecture
clearly. It is a basic step or main point that you need before you go to manage
your database. By this article, I will try to share my knowledge about it. Hope
it can be useful for you.
What is An Oracle Database?
Basically, there are two main components of Oracle database 末 instance and
database itself. An instance consists of some memory structures and the
background processes, whereas a database refers to the disk resources. Figure 1
will show you the relationship.
Figure 1. Two main components of Oracle database
Instance
As we cover above, the memory structures and background processes contitute an
instance. The memory structure itself consists of System Global Area (SGA),
Program Global Area (PGA), and an optional area 末 Software Area Code. In the
other hand, the mandatory background processes are Database Writer (DBWn), Log
Writer (LGWR), Checkpoint (CKPT), System Monitor (SMON), and Process Monitor (PMON).
And another optional background processes are Archiver (ARCn), Recoverer (RECO),
etc. Figure 2 will illustrate the relationship for those components on an
instance.
Figure 2. The instance components
System Global Area
SGA is the primary memory structures. When Oracle DBAs talk about memory, they
usually mean the SGA. This area is broken into a few of part memory 末 Buffer
Cache, Shared Pool, Redo Log Buffer, Large Pool, and Java Pool.
Buffer Cache
Buffer cache is used to stores the copies of data block that retrieved from
datafiles. That is, when user retrieves data from database, the data will be
stored in buffer cache. Its size can be manipulated via DB_CACHE_SIZE parameter
in init.ora initialization parameter file.
Shared Pool
Shared pool is broken into two small part memories 末 Library Cache and
Dictionary Cache. The library cache is used to stores information about the
commonly used SQL and PL/SQL statements; and is managed by a Least Recently Used
(LRU) algorithm. It is also enables the sharing those statemens among users. In
the other hand, dictionary cache is used to stores information about object
definitions in the database, such as columns, tables, indexes, users,
privileges, etc.
The shared pool size can be set via SHARED_POOL_SIZE parameter in init.ora
initialization parameter file.
Redo Log Buffer
Each DML statement (insert, update, and delete) executed by users will generates
the redo entry. What is a redo entry? It is an information about all data
changes made by users. That redo entry is stored in redo log buffer before it is
written into the redo log files. To manipulate the size of redo log buffer, you
can use the LOG_BUFFER parameter in init.ora initialization parameter file.
Large Pool
Large pool is an optional area of memory in the SGA. It is used to relieves the
burden place on the shared pool. It is also used for I/O processes. The large
pool size can be set by LARGE_POOL_SIZE parameter in init.ora initialization
parameter file.
Java Pool
As its name, Java pool is used to services parsing of the Java commands. Its
size can be set by JAVA_POOL_SIZE parameter in init.ora initialization parameter
file.
Program Global Area
Although the result of SQL statemen parsing is stored in library cache, but the
value of binding variable will be stored in PGA. Why? Because it must be private
or not be shared among users. The PGA is also used for sort area.
Software Area Code
Software area code is a location in memory where the Oracle application software
resides.
Oracle Background Processes
Oracle background processes is the processes behind the scene that work together
with the memories.
DBWn
Database writer (DBWn) process is used to write data from buffer cache into the
datafiles. Historically, the database writer is named DBWR. But since some of
Oracle version allows us to have more than one database writer, the name is
changed to DBWn, where n value is a number 0 to 9.
LGWR
Log writer (LGWR) process is similar to DBWn. It writes the redo entries from
redo log buffer into the redo log files.
CKPT
Checkpoint (CKPT) is a process to give a signal to DBWn to writes data in the
buffer cache into datafiles. It will also updates datafiles and control files
header when log file switch occurs.
SMON
System Monitor (SMON) process is used to recover the system crach or instance
failure by applying the entries in the redo log files to the datafiles.
PMON
Process Monitor (PMON) process is used to clean up work after failed processes
by rolling back the transactions and releasing other resources.
Database
The database refers to disk resources, and is broken into two main structures 末
Logical structures and Physical structures.
Logical Structures
Oracle database is divided into smaller logical units to manage, store, and
retrieve data effeciently. The logical units are tablespace, segment, extent,
and data block. Figure 3 will illustrate the relationships between those units.
Figure 3. The relationships between the Oracle logical structures
Tablespace
A Tablespace is a grouping logical database objects. A database must have one or
more tablespaces. In the Figure 3, we have three tablespaces 末 SYSTEM
tablespace, Tablespace 1, and Tablespace 2. Tablespace is composed by one or
more datafiles.
Segment
A Tablespace is further broken into segments. A segment is used to stores same
type of objects. That is, every table in the database will store into a specific
segment (named Data Segment) and every index in the database will also store in
its own segment (named Index Segment). The other segment types are Temporary
Segment and Rollback Segment.
Extent
A segment is further broken into extents. An extent consists of one or more data
block. When the database object is enlarged, an extent will be allocated. Unlike
a tablespace or a segment, an extent cannot be named.
Data Block
A data block is the smallest unit of storage in the Oracle database. The data
block size is a specific number of bytes within tablespace and it has the same
number of bytes.
Physical Structures
The physical structures are structures of an Oracle database (in this case the
disk files) that are not directly manipulated by users. The physical structure
consists of datafiles, redo log files, and control files.
Datafiles
A datafile is a file that correspondens with a tablespace. One datafile can be
used by one tablespace, but one tablespace can has more than one datafiles.
Redo Log Files
Redo log files are the files that store the redo entries generated by DML
statements. It can be used for recovery processes.
Control Files
Control files are used to store information about physical structure of
database, such as datafiles size and location, redo log files location, etc.
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