Oracle Multiple Instances

From your posting I get the idea that you are relatively new to Oracle.  I'll tackle answering your question from that perspective.  (If I'm wrong, you may want to rephrase the question to be a little bit more specific.)  The 'default database' that gets installed is in part used by Oracle to maintain the database - user accounts, table definitions, privileges, performance stats, etc.  Creating another instance would give you an (almost) identicle setup to what you already have.  What you are probably really asking is how to set up an area inside of that instance to play with (created/drop tables, insert/update/delete records, etc).  All you really need to do is create yourself a user account, assign your user account a default tablespace, grant your useraccount a quota on that tablespace, etc.  Then you can start to create tables, etc under that user account without affecting Oracle's internal workings too much.  I would highly recommend going to the bookstore and purchasing one of the books from the OraclePress series.  I've used many of them and I have yet to be disappointed with them.  As far as OS settings, I assume that you have the database up and running already which means that you have already made corrections to the /etc/system file for shared memory.  Until you really start to understand how all of the background processes os OS interact, those settings are sufficient for the time being.

In answering your database size question:  You are only limited by the size of your drive or drives mounted on your Solaris box.  You and add or 'grow' tablespaces until you run out of drive space.

Hegdie is new to oracle and I am not new to oracle.O.K.I asked how to start another instance in the same machine

I am by far not new to Oracle.  Your question however is very vague and leaves out a lot of important information:  Is the first instance up and properly running without any problems?  If not, what problems are you experiencing with it (these need to be cleared up before getting the second instance up and running)?

Is there a specific reason for getting a second instance running on the same machine that may be more easily (and less resource draining) handled other than going through the necessary hassle of configuring for a second instance?

If you have started setting up the second instance, how far are you?  Is it up and running? Have you created the 2nd init.ora file?  Added the new instance to the oratab file?  Created the proper environment variables?  Configured the tnslistener for the new instance?  Run the proper scripts to create the data dictionary etc?

I am also not new to Oracle, since I require a second database installed in my machine , I asked that question Mr.Hegdie.O.K.Don't judge from the way question is asked.Try to answer if possible, else keep quiet

VRamachan,
   
If you would like to create a second Database on the machine , here are the steps you
need to follow -

Adding a second Database on to the existing configuration of your machine, is
going to increase the requirements on all sides you need more memory, more disk etc.

Memory Requirements & Kernel parameters -

On Solaris boxes, the Semaphore information is held in /etc/system file -
The parameters that you are interested are
shmmax , shmmin, semmni, semmns .

I have two databases on a particular solaris machine with the following memory parameters -
set shmsys:shminfo_shmmax=33556432
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=100
set shmsys:shminfo_shmseg=10
set semsys:seminfo_semmns=200
set semsys:seminfo_semmni=70

Please look up Oracle Documentation for these - This would be in the Unix
administrator Guide that came along with the installation manual.

Amongst these the most important one is shmmax. This is the biggest chunk of
contiguous memory that can be allocated. This is the area of the SGA .
this means the variable portion cannot be larger than 33.5 MB.
So please set this such that you are able to get a decently sized SGA.

This amount of Physical memory is preallocated and absorbed by oracle and is not
available to the other processes. while sizing the memory kernel parameters ,
One thing to remember - This you must follow pretty strict, there should be somewhere between
5-10% of Physical memory free at all times.


Regarding Disk Requirements -
2.1 GB - is this disk available for your second instance. Depending on the activity you
expect to have on your database , this may or may not be an ideal situation.

As a general rule - you must try and split the differnt tablespaces as much as possible on
to different disks, to reduce contention.

The things that are best placed on different disks are
Data, Indexes, redo logs .
 
Please make sure you set some init.ora parameters such as processes , sessions , open cursors
enqueue resources  and transactions,

IF THIS COMMENT  SATISFIES YOUR QUERY, PLEASE  ALLOW ME TO POST THIS AS ANSWER.

If you require more info, please post.
Thanks.











VRamachan,
   
If you would like to create a second Database on the machine , here are the steps you
need to follow -

Adding a second Database on to the existing configuration of your machine, is
going to increase the requirements on all sides you need more memory, more disk etc.

Memory Requirements & Kernel parameters -

On Solaris boxes, the Semaphore information is held in /etc/system file -
The parameters that you are interested are
shmmax , shmmin, semmni, semmns .

I have two databases on a particular solaris machine with the following memory parameters -
set shmsys:shminfo_shmmax=33556432
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=100
set shmsys:shminfo_shmseg=10
set semsys:seminfo_semmns=200
set semsys:seminfo_semmni=70

Please look up Oracle Documentation for these - This would be in the Unix
administrator Guide that came along with the installation manual.

Amongst these the most important one is shmmax. This is the biggest chunk of
contiguous memory that can be allocated. This is the area of the SGA .
this means the variable portion cannot be larger than 33.5 MB.
So please set this such that you are able to get a decently sized SGA.

This amount of Physical memory is preallocated and absorbed by oracle and is not
available to the other processes. while sizing the memory kernel parameters ,
One thing to remember - This you must follow pretty strict, there should be somewhere between
5-10% of Physical memory free at all times.


Regarding Disk Requirements -
2.1 GB - is this disk available for your second instance. Depending on the activity you
expect to have on your database , this may or may not be an ideal situation.

As a general rule - you must try and split the differnt tablespaces as much as possible on
to different disks, to reduce contention.

The things that are best placed on different disks are
Data, Indexes, redo logs .
 
Please make sure you set some init.ora parameters such as processes , sessions , open cursors
enqueue resources  and transactions,

IF THIS ANSWERS YOU, I SHALL POST THIS AS ANSWER.

If you require more info, please post.
Thanks.

P.S After Changing the system parameters in the /etc/system file, you'd need to reboot your system for it to take effect.
Thanks.