Databases Reference
In-Depth Information
Advantages
The easiest advantage to understand is the visibility across all instances of a real application cluster (RAC) database.
Since the fast recovery area must be visible to all the nodes of the RAC database (Recipe 3-1), it makes a perfect
location for a control file, which must be visible to all the nodes as well. Only one control file should be placed there.
The rest should be placed in other locations.
It's the other advantage that is more significant, one that relates to availability. If their primary database fails and
you need to recover (or restore, whatever is appropriate), the fast recovery area is used, which has the backups of the
database. So, technically, you have positioned the fast recovery area on such areas of the storage that the placement
reduces the probability of failure at the same time as the failure of the primary database. For instance, you may have
put your database disks on a SAN different from where the fast recovery area disks are. So, the chances of both disks
(the database and the fast recovery area) going down at the same time are substantially reduced. If the primary
database files are down and you have access to the most current online redo log files, you may avoid the possibility of
an incomplete recovery, since you have a control file and will not need to start a recovery using a backup control file.
If you don't have a control file on the fast recovery area, then there is a fair chance you will have to resort to a backup
control file during recovery, which means an incomplete recovery, even if you have access to the current redo log files.
So, there is a strong argument for placing one control file in the fast recovery area.
Disadvantages
It's not a slam-dunk argument; there is a significant disadvantage that you should consider. In a more practical
situation, you probably have limited resources (read: money to buy disks) and want to maximize your investment
for performance and reliability. So, you probably made the storage location of the main database files on RAID level
0+1, have a more reliable SAN, and so on. And you may have placed the fast recovery area on a less reliable (and
less expensive) storage area network (SAN), even on network attached storage (NAS), and perhaps with RAID 5 or
even no RAID at all. The latter is not advisable but is not unusual. So, the chance of failure in the fast recovery area
is greater compared to that of the main database disks. If the fast recovery area fails, then you lose one of the control
files. This by itself is not the end of the world. Let's hope you have been prudent in putting the other control files in
other locations. So, when the database comes down after the control file in the fast recovery area suddenly becomes
inaccessible, all you have to do is to remove that control file from the control file parameter in the initialization
parameter file and restart the database. There is no data loss; you will have an interruption of service, since the
database is unavailable from the time the fast recovery area is unavailable and the database is back up after removing
the control file from the initialization parameter.
Choice
Here comes the tough question: should you put a control file in the fast recovery area? If your fast recovery area is in a
storage location as reliable as the main database storage, then we strongly urge you to put one control file there.
If that is not true (and most likely the case), decide how important complete recovery is to you. If you must have
a complete recovery after a failure regardless of other consequences including a possible interruption in service, then
put one control file in the fast recovery area. If the potential service interruptions in case of the fast recovery area
failure are not acceptable, do not use it as a location for even one control file.
Without knowing your exact circumstances, it's not easy for us to recommend one solution over the other.
In general, however, we find it safer not to put even one control file in the fast recovery area. Under no circumstances
should you put all your control files in the fast recovery area.
