Re: Meaning of an INSTANCE.

Nabil Courdy wrote:
>
> I am having trouble with the meaning of an instance. If
> I have a very busy production machine, does it make sense
> to have more than one instance? What I am trying to say is,
> for each database, say: payroll, manufacturing, and hr,
> do I have to have three instances, or somehow I create
> all three databases into one instance? Because, if I have
> to have three instances, that seems like a lot of processes
> running to service them.
>
> Thx.

A database is a collection of Oracle control, redo log, init.ora (config.ora) and data disk files.

An instance is an executing set of processes with an SGA reading and writing to a database.

In a parallel server you could have several instances accessing the same set of database files, but each would have its own SGA (and activities would have to be co-ordinated through a shared locking mechanism).

In your case you are considering having 3 databases. When they are all running you will have 3 instances.

There a number of trade offs (Aren't there always?)

If you split your applications into separate database you can tune the rollback segments, temporary tablespace, SGA size and so on to your particular application. You can separate each one to its own listener and control client accesses, you can shut them down separately and back them up to different schedules. You can have stronger security if you have disjoint user groups. If you don't have the horsepower on one server you can get another server and move one or two databases to it. (Or two more servers, of course...)

If you put them together you will be able to pool the overhead of rollback segments, temporary tablespace and SGA and you may be able to get as much done with less overall resources in these categories. You may not particularly want to separate the listeners.

You may find it easier to manage one larger database, one set of archived redo log files, one set of backups etc. etc. than 3 separate ones.

If you go with the multiple databases you may have to modify your OS limit on processes per user.

I am sure others in the group will fill in the gaps I may have left in haste. Thanks to them in advance....

Marek Received on Mon Nov 04 1996 - 00:00:00 CET