Oracle Database Discussions

I installed Oracle 10g on CentOS Linux, run netca to create the listener.ora and tnsnames.ora files, then I run dbca to create database myodb and an instance orcl.



Now when I run dbstart, in the startup log it says that it couldn't find init file for instance orcl. while for the myodb database it says:

<em>/oracle/product/10.2.0/db_1/bin/dbstart: Database instance "myodb" warm started.





</em>My questions:



1- why it says when i run dbstart that both, myodb & orcl are database instances, I created myodb as the general database, while orcl is the instance, here what it says:

<em>Processing Database instance "orcl": log file /oracle/product/10.2.0/db_1/startup.log

Processing Database instance "myodb": log file /oracle/product/10.2.0/db_1/startup.log



</em>Is the general database name is considered an instance itself and can be used alternatively with the word instance?



2- When I created the database using dbca, i made the password for SYSTEM & SYS users.



When I try to connect to sqlplus using system/mypassword, here what it says:



<strong><em>ORA-01034: ORACLE not available

ORA-27101: shared memory realm does not exist

Linux Error: 2: No such file or directory</em></strong>



3- The database myodb started though it's not mentioned in my tnsnames or the listener, while the one mentioned which is orcl didn't start, how come? I know i am missing a dir for /orcl in /oracle/home/admin, a thing which is found for the myodb, where there is /oracle/home/admin/myodb/pfile/init file.



How can I correct this?



4- What is the use for the sql command, <strong>alter system register</strong>, i was told in another thread to connect to idle instance with sqlplus, and run this command, here what i got:



<em>ERROR at line 1:

ORA-01034: ORACLE not available



</em>5- Can I just depend on the general database name and forget about the instance thing? I am just confused about the difference, and not sure how the orcl instance was created, it wasn't created with dbca, because when i run dbca it said it's already there, create another instance, so i commented the sid paramater, and now I amn't sure when it was created, and how to get back its files if I need to re-create it again.



Note: it's set in the bash_profile as $ORACLE_SID=orcl, why it's the only one set, while the general database name MYODB isn't set, can be there more than on instance or SID in the bash_profile?





Sorry for the many questions, it's all about the instance thing and their related subject, I appreciate your help.



Thanks in advance.