Files that control oracle instances

Oracle database is comprised of a set of operating system files containing data entered by users or applications and structural information about the database itself called database metadata. Information is stored persistently in these files.Oracle needs to start a set of processes, called background processes, and needs to allocate some memory to be used during database operation. The background processes and memory allocated by Oracle together make up an instance. Therefore before the database can be used, the database instance must be started. When the database instance is not available, your data is safe in the database but it cannot be accessed by any user or application. The Files That Control Such Instances are

Parameter Files And Password Files.

What is a parameter File In Oracle???

This is a plain text file which consist of various parameters and a value associated with these parameter which control or configure the Oracle Instance.There are alomost over 200 configurable parameters and a default value is set for each of these parameters.The Non default values are generally the content which is available in the parameter file. When You make a changes inside the parameter file the new values of the parameter will not effect the oracle instance untill unless you shut down oracle and re start it.

What Role Does A Parameter File Has Oracle???

When you start an Oracle instance, Oracle looks for a parameter file named initSID.ora (where SID is the name of the instance) and uses the parameter file to configure the instance. parameter files are read once when an instance is started.

What Does A Parameter File Specify???

The parameter file specifies such things as

Location and name of the database to mount
How much memory to allocate for a buffer cache
Maximum number of processes that will be able to access the instance.

Do Note different Version of Oracle Higer Versions may have some parameters which are not available with the lower Versions

SAMPLE PARAMETER FILE

# initpubdb.ora
# Parameter file for pubdb database.
# config parameters in the oracle parameter file
# Please note the values specified in the following prameters are just hypothetical
# figure to explain the content of Parameter file and is suggest not to use these
# values in your existing parameter file or without the consent of your DBA

control_files = (/u02/oradata/pubdb/control01.ctl,/u03/oradata/pubdb/control02.ctl,/u04/oradata/pubdb/control03.ctl)
rollback_segments = (r01,r02,r03,r04)
background_dump_dest = /u01/app/oracle/admin/pubdb/bdump
core_dump_dest = /u01/app/oracle/admin/pubdb/cdump
user_dump_dest = /u01/app/oracle/admin/pubdb/udump
utl_file_dir = /u01/app/oracle/admin/pubdb/bdump
log_archive_dest = /u01/app/oracle/admin/pubdb/bk/arch
db_name = pubdb
db_files = 80

Above parameters are various location and name of various databse file 

db_block_size = 8192
compatible = 8.0.5
log_archive_format = %S.log
log_archive_start = true

Tuning parameters. Before you work with the sizes check your Hardware memory resources.
Novice & Self Proclaimed DBA's are advised not to play with the numbers specified in the Sample Parameter Files
Following values are hypothetical figures and is not a value of any online live oracle database

shared_pool_size = 16000000
sort_area_size = 4096
db_block_buffers = 2048
db_file_multiblock_read_count = 128
processes = 100
sessions = 100
log_checkpoint_interval = 10000
log_buffer = 163840
sequence_cache_entries = 100
sequence_cache_hash_buckets = 89
job_queue_processes = 2
max_dump_file_size = 10240

When Oracle Parallel Server is not being used, all of the instance parameter settings should be stored in one parameter file called initSID.ora. In case of a Oracle Parallel Server in use, some parameter settings will need to be the same for all instances mounting the Same database, while others can vary from instance to instance. Therefore, when using Oracle Parallel Server, each instance should have its own parameter file called initSID.ora containing Settings for that intance only, and all initSID.ora files should reference one common config.ora file that contains the Settings that need to be the same for all instances.

PASSWORD FILE

This is the another file that can have a control over oracle instances.
A password file indicates to the instance the usernames and passwords of people authorized to start up and shut down the  instance. Once a database is open, a table in the databases data dictionary is used to authenticate users wishing to access the database. However, this doesn't help much if the database has not been opened yet. This is where the password file comes in.

A password file is often not used in smaller shops running Oracle on Unix, because there is an alternative. Instead of checking a password file to see if a user is authorized to start the instance, Oracle can ask the operation system to authenticate the user. On Unix platforms this amounts to seeing if the user belongs to a special Unix group, typically called Dba. The net result is that anybody with a Unix login that belongs to the dba group can start up and shut down all instances running on the server.

Password files are sometimes used in larger shops that have many databases. In such shops the DBAs may use an Oracle tool
called Enterprise Manager which allows you to start up and shut down databases from a central location. In this arrangement, the DBA never logs on to the actual server where the instance is located, so operating system authentication cannot be used.
Hence the need for a password file.