INTRODUCTION============This article discusses the most common
concepts of backup andrecovery of the Oracle database.This article
discusses physical backups and not loical backups.!"n e#port of the
database is an e#ample of a loical backup.$%hy do &e need
backups'======================The most important responsibility of
a database administrator is toprepare for the possibility of media(
hard&are and soft&are failure.)hould any of these failures
occur the ma*or oal is to make thedatabase available to users
&ithin an acceptable time( ensurinthat committed data is
undamaed.This paper discusses only physical backups and not loical
backups."n e#port of the database is an e#ample of a loical
backup.%hat are the thins that o &ron and eventually lead to
recovery'=================================================================One
or more database +les are damaed.One or more redo lo +les !includin
online$ are damaed.One or all control +les are damaed.%hat is
recovery'================Restorin the +les from backup and rollin
for&ard in time is R,CO-,R.."N INT,RN"/
-I,%================)ome admnistrative actions can destroy a
database beyond repair. This isparticularly true of backup and
recovery commands. To understand thedaners of certain commands( a
database administrator should have aood understandin of the
contents and purpose of the control +le(data+les and lo +les. The
basics are provided here.Control +le============" control +le
re0ects the structure of a database at particular pointsin time. It
contains the checkpoint information( names of lo +lesand data +les(
header information of the +les and lo se1uence number(&hich is
very important for recovery purposes. The recovery is doneonly by
applyin the lo +les &hose se1uence number is reater thanlo
se1uence number in the control +le.Data+le information in control
+le23333333333333333333333333333333333333 Names of data+les and lo
+les &ith e#act path.3 4ile si5e.3 6lock si5e.!Oracle block
si5e$3 %hether the data+le is online or o7ine.3 %hether the data+le
&as taken o7ine automatically or not.3 %hether the data+le
belons to the system tablespace or not.3 ,ntry for each data+le
ivin a lo se1uence number &hen tablespace&as taken o7ine./o
+le information in control
+le23333333333333333333333333333333333333 Name &ith e#act
path.3 4ile si5e.3 6lock si5e. !O8) block si5e$3 /o se1uence93 :as
the +le been archived.Information in the data+le
header==================================3 /o se1uence number of
ne#t lo +le that could be applied.3 %hether online backup in
proress.Information in the lo +le
header==================================3 /o se1uence93 "rchival
information.6"C;U?3hours per day and &hen it isnot feasible to
take o7ine backups( then an alternative is providedby Oracle
&here physical backups are performed &hile the database
remainsavailable for both readin and updatin. 4or this kind of
backup thedatabase must be in "RC:I-,/O= mode. Only data +les and
currentcontrol +le need to be backed up. Unlike o7ine backups( the
unit ofa online backup is a tablespace( and any or all tablespaces
can backedup &henever needed. Di@erent data+les can be backed
up at di@erenttimes.
