1.How to check whether dataguard broker is enabled or not?Any command to check?
show parameter dg_broker_start
if this parameter is TRUE, it means DG Broker is started DMON is runinig.
2.What is the use of dataguard broker?and how it works?
DGMGRL> show configuration
if showed any dg configuration, it means broker - managed DG configuration s created.
DG Broker is very useful for managing data guard. You can manage data guard with DGMGRL (Data Guard Manager Commad Line) tool
http://www.mahir-quluzade.com/p/video-tutorials.html On this page, Data Guard Administration part
have many videos about administration of data guard with Data Guard Broker.
3.I have to do Auditing at user level trace in production database.what are the pre-requisits for it?
What are the Steps to do user level auditing?
Any idea how much disk utilization after enabling auditing?
You can use Fine-Grained Auditing for user.