Set - 4

Question 11 :

What is meant by Redo Log file mirroring ? How it can be achieved?

Answer :

Process of having a copy of redo log files is called mirroring. This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the members of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance.


Question 12 :

Which parameter in Storage clause will reduce no. of rows per block?

Answer :

PCTFREE parameter
Row size also reduces no of rows per block.


Question 13 :

What is meant by recursive hints ?

Answer :

Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache.


Question 14 :

What is the use of PARFILE option in EXP command ?

Answer :

Name of the parameter file to be passed for export.


Question 15 :

What is the difference between locks, latches, enqueues and semaphores? (for DBA)

Answer :

A latch is an internal Oracle mechanism used to protect data structures in the SGA from simultaneous access. Atomic hardware instructions like TEST-AND-SET is used to implement latches. Latches are more restrictive than locks in that they are always exclusive. Latches are never queued, but will spin or sleep until they obtain a resource, or time out.
Enqueues and locks are different names for the same thing. Both support queuing and concurrency. They are queued and serviced in a first-in-first-out (FIFO) order.
Semaphores are an operating system facility used to control waiting. Semaphores are controlled by the following Unix parameters: semmni, semmns and semmsl. Typical settings are:
semmns = sum of the "processes" parameter for each instance
(see init.ora for each instance)
semmni = number of instances running simultaneously;
semmsl = semmns