Database Reference
In-Depth Information
Prior to the
FLASHBACK DATABASE
capability, what would probably happen is this:
1.
The DBA would shut down the database.
2.
The DBA would restore the last full backup of database from tape (typically), generally a
long process. Typically this would be initiated with RMAN via
RESTORE DATABASE UNTIL
<point in time>
.
3.
The DBA would restore all archive redo logs generated since the backup that were not
available on the system.
4.
Using the archive redo logs (and possibly information in the online redo logs), the DBA
would roll the database forward and stop rolling forward at a point in time just before the
erroneous
DROP USER
command. Steps 3 and 4 in this list would typically be initiated with
RMAN via
RECOVER DATABASE UNTIL <point in time>
.
5.
The database would be opened with the
RESETLOGS
option.
This was a nontrivial process with many steps and would generally consume a large piece of time (time when no
one could access the database, of course). The causes of a point-in-time recovery like this are many: an upgrade
script gone awry, an upgrade gone bad, an inadvertent command issued by someone with the privilege to issue it
(a mistake, probably the most frequent cause), or some process introducing data integrity issues into a large database
(again, an accident; maybe it was run twice instead of just once, or maybe it had a bug). Whatever the reason, the net
effect was a large period of downtime.
The steps to recover in Oracle 10
g
Enterprise Edition and above, assuming you configured the flashback database
capability, would be as follows:
1.
The DBA shuts down the database.
2.
The DBA startup-mounts the database and issues the flashback database command, using
either an SCN (the Oracle internal clock), a restore point (which is a pointer to an SCN), or
a timestamp (wall clock time), which would be accurate to within a couple of seconds.
3.
The DBA opens the database with resetlogs.
To use this feature, the database must be in
ARCHIVELOG
mode and must have been set up to enable the
FLASHBACK DATABASE
command. What I'm trying to say is that you need to set up this capability before you ever need
to use it. It is not something you can enable after the damage is done; you must make a conscious decision to use it,
whether you have it on continuously or whether you use it to set restore points.
Fast Recovery Area
The Fast Recovery Area is a new concept in Oracle 10
g
and above. Starting with 10
g
and for the first time in many years
(over 25 years), the basic concept behind database backups has changed in Oracle. In the past, the design of backup
and recovery in the database was built around the concept of a sequential medium, such as a tape device. That is,
random access devices (disk drives) were always considered too expensive to waste for mere backups. You used
relatively inexpensive tape devices with large storage capacities.
Today, however, you can buy terabytes of disk storage at a very low cost. In fact, my son Alan was the first kid on
the block with a 1TB NAS (network-attached storage device). It cost $125.00 USD. I remember my first hard drive on
my personal computer: a whopping 40MB. I actually had to partition it into two logical disks because the OS I was
using (MS-DOS at the time) could not recognize a disk larger than 32MB. Things have certainly changed in the last 25
years or so.
