Backup and Recovery Scenariosa) Consistent backups
A consistent backup means that all data files and control files are consistent to a point in time. I.e. they have the same SCN. This is the only method of backup when the database is in NO Archive log mode.b) Inconsistent backups
An Inconsistent backup is possible only when the database is in Archivelog mode. We must apply redo logs to the data files, in order to restore the database to a consistent state. Inconsistent backups can be taken using RMAN when the database is open.
Inconsistent backups can also be taken using other OS tools provided the tablespaces (or database) is put into backup mode.
ie: SQL> alter tablespace data begin backup;
SQL> alter database begin backup; (version 10 and above only)
c) Database Archive mode
The database can run in either Archivelog mode or noarchivelog mode. When we first create the database, we specify if it is to be in Archivelog mode. Then in the init.ora file we set the parameter log_archive_start=true so that archiving will start automatically on startup.
If the database has not been created with Archivelog mode enabled, we can issue the command whilst the database is mounted, not open.
SQL> alter database Archivelog;.
SQL> log archive start
SQL> alter database open;
SQL> archive log list
This command will show us the log mode and if automatic archival is set.
d) Backup Methods
Essentially, there are two backup methods, hot and cold, also known as online and offline, respectively. A cold backup is one taken when the database is shutdown. The database must be shutdown cleanly. A hot backup is on taken when the database is running. Commands for a hot backup:
For non RMAN backups:
1. Have the database in archivelog mode (see above)
2. SQL> archive log list
--This will show what the oldest online log sequence is. As a precaution, always keep the all archived log files starting from the oldest online log sequence.
3. SQL> Alter tablespace tablespace_name BEGIN BACKUP;
or SQL> alter database begin backup (for v10 and above).
4. --Using an OS command, backup the datafile(s) of this tablespace.
5. SQL> Alter tablespace tablespace_name END BACKUP
--- repeat step 3, 4, 5 for each tablespace.
or SQL> alter database end backup; for version 10 and above
6. SQL> archive log list
---do this again to obtain the current log sequence. make sure that we have a copy of this redo log file.
7. So to force an archived log, issue
SQL> ALTER SYSTEM SWITCH LOGFILE
A better way to force this would be:
SQL> alter system archive log current;
8. SQL> archive log list
This is done again to check if the log file had been archived and to find the latest archived sequence number.
9. Backup all archived log files determined from steps 2 and 8.
10. Back up the control file:
SQL> Alter database backup controlfile to 'filename'
For RMAN backups:
see Note.<> RMAN - Sample Backup Scripts 10g
or the appropriate RMAN documentation.
e) Incremental backups
These are backups that are taken on blocks that have been modified since the last backup. These are useful as they don't take up as much space and time. There are two kinds of incremental backups Cumulative and Non cumulative.
Cumulative incremental backups include all blocks that were changed since the last backup at a lower level. This one reduces the work during restoration as only one backup contains all the changed blocks.
Noncumulative only includes blocks that were changed since the previous backup at the same or lower level.
Using rman, we issue the command "backup incremental level n"
Oracle v9 and below RMAN will back up empty blocks, oracle v10.2 RMAN will not back up empty blocks
f) Support scenarios
When the database crashes, we now have a backup. We restore the backup and
then recover the database. Also, don't forget to take a backup of the control
file whenever there is a schema change.
RECOVERY SCENARIOS
Note: All online datafiles must be at the same point in time when completing recovery;There are several kinds of recovery we can perform, depending on the type of failure and the kind of backup we have. Essentially, if we are not running in archive log mode, then we can only recover the cold backup of the database and we will lose any new data and changes made since that backup was taken. If, however, the database is in Archivelog mode we will be able to restore the database up to the time of failure. There are three basic types of recovery:
1. Online Block Recovery.
This is performed automatically by Oracle.(pmon) Occurs when a process dies while changing a buffer. Oracle will reconstruct the buffer using the online redo logs and writes it to disk.
2. Thread Recovery.
This is also performed automatically by Oracle. Occurs when an instance crashes while having the database open. Oracle applies all the redo changes in the thread that occurred since the last time the thread was checkpointed.
3. Media Recovery.
This is required when a data file is restored from backup. The checkpoint count in the data files here are not equal to the check point count in the control file.
Now let's explain a little about Redo vs Undo.
Redo information is recorded so that all commands that took place can be repeated during recovery. Undo information is recorded so that we can undo changes made by the current transaction but were not committed. The Redo Logs are used to Roll Forward the changes made, both committed and non- committed changes. Then from the Undo segments, the undo information is used to
rollback the uncommitted changes.
Media Failure and Recovery in Noarchivelog Mode
In this case, our only option is to restore a backup of Oracle files. The files we need are all datafiles, and control files. We only need to restore the password file or parameter files if they are lost or are corrupted.
Media Failure and Recovery in Archivelog Mode
In this case, there are several kinds of recovery we can perform, depending on what has been lost.
The three basic kinds of recovery are:
1. Recover database - here we use the recover database command and the database must be closed and mounted. Oracle will recover all datafiles that are online.
2. Recover tablespace - use the recover tablespace command. The database can be open but the tablespace must be offline.
3. Recover datafile - use the recover datafile command. The database can be open but the specified datafile must be offline.
Note: We must have all archived logs since the backup we restored from, or else we will not have a complete recovery.
a) Point in Time recovery:
A typical scenario is that we dropped a table at say noon, and want to recover it. We will have to restore the appropriate datafiles and do a point-in-time recovery to a time just before noon.
Note: We will lose any transactions that occurred after noon. After we have recovered until noon, we must open the database with resetlogs. This is necessary to reset the log numbers, which will protect the database from having the redo logs that weren't used be applied.
The four incomplete recovery scenarios all work the same:
Recover database until time '1999-12-01:12:00:00';
Recover database until cancel; (we type in cancel to stop)
Recover database until change n;
Recover database until cancel using backup controlfile;
Note: When performing an incomplete recovery, the datafiles must be online. Do a select * from v$recover_file to find out if there are any files which are offline. If we were to perform a recovery on a database which has tablespaces offline, and they had not been taken offline in a normal state, we will lose them when we issue the open resetlogs command. This is because the data file needs recovery from a point before the resetlogs option was used.
b) Recovery without control file
If we have lost the current control file, or the current control file is inconsistent with files that we need to recover, we need to recover either by using a backup control file command or create a new control file. We can also recreate the control file based on the current one using the 'SQL> backup control file to trace' command which will create a script for we to run to create a new one. Recover database using backup control file command must be used when using a control file other that the current. The database must then be opened with
resetlogs option.
c) Recovery of missing datafile with rollback segments
The tricky part here is if we are performing online recovery. Otherwise we can just use the recover datafile command. Now, if we are performing an online recovery, we will need to create a new undo tablespace to be used. Once the old tablespace has been recovered it can be dropped once any uncommitted transactions have rolled back.
d) Recovery of missing datafile without undo segments
There are three ways to recover in this scenario, as mentioned above.
1. recover database;
2. recover datafile 'c:\orant\database\usr1orcl.ora';
3. recover tablespace user_data;
e) Recovery with missing online redo logs
Missing online redo logs means that somehow we have lost our redo logs before they had a chance to archived. This means that crash recovery cannot be performed, so media recovery is required instead. All datafiles will need to be restored and rolled forwarded until the last available archived log file is applied. This is thus an incomplete recovery, and as such, the recover
database command is necessary.
As always, when an incomplete recovery is performed, we must open the database with resetlogs.
Note: the best way to avoid this kind of a loss, is to mirror online log files.
f) Recovery with missing archived redo logs
If archives are missing, the only way to recover the database is to restore from latest backup. We will have lost any uncommitted
transactions which were recorded in the archived redo logs. Again, this is why Oracle strongly suggests mirroring online redo logs and duplicating copies of the archives.
g) Recovery with resetlogs option
Reset log option should be the last resort, however, as we have seen from above, it may be required due to incomplete recoveries. (recover using a backup control file, or a point in time recovery). It is imperative that we backup up the database immediately after we have opened the database with reset logs. It is possible to recover through a resetlogs, and made easier with Oracle V10, but easier
to restore from the backup taken after the resetlogs
h) Recovery with corrupted undo segments.
If an undo segment is corrupted, and contains uncommitted system data we may not be able to open the database.
The best alternative in this situation is to recover the corrupt block using the RMAN blockrecover command next best would be to restore the datafile from backup and do a complete recovery.
If a backup does not exist and If the database is able to open (non system object) The first step is to find out what object is causing the rollback to appear corrupted. If we can determine that, we can drop that object.
So, how do we find out if it's actually a bad object?
1. Make sure that all tablespaces are online and all datafiles are online. This can be checked through via the v$recover_file view.
2. Put the following in the init.ora:
event = "10015 trace name context forever, level 10"
This event will generate a trace file that will reveal information about the transaction Oracle is trying to roll back and most importantly, what object Oracle is trying to apply the undo to.
Note: In Oracle v9 and above this information can be found in the alert log.
Stop and start the database.
3. Check in the directory that is specified by the user_dump_dest parameter (in the init.ora or show parameter command) for a trace file that was generated at startup time.
4. In the trace file, there should be a message similar to: error recovery tx(#,#) object #.
TX(#,#) refers to transaction information.
The object # is the same as the object_id in sys.dba_objects.
5. Use the following query to find out what object Oracle is trying to perform recovery on.
select owner, object_name, object_type, status
from dba_objects where object_id = <object #>;
6. Drop the offending object so the undo can be released. An export or relying on a backup may be necessary to restore the object after the corrupted undo segment is released.
i) Recovery with System Clock change.
We can end up with duplicate timestamps in the datafiles when a system clock changes. This usually occurs when daylight saving comes into or out of the picture. In this case, rather than a point in time recovery, recover to a specify log or SCN
j) Recovery with missing System tablespace.
The only option is to restore from a backup.
k) Media Recovery of offline tablespace
When a tablespace is offline, we cannot recover datafiles belonging to this tablespace using recover database command. The reason is because a recover database command will only recover online datafiles. Since the tablespace is offline, it thinks the datafiles are offline as well, so even if we recover database and roll forward, the datafiles in this tablespace will not be touched. Instead, we need to perform a recover tablespace command. Alternatively, we could restored the datafiles from a cold backup, mount the database and select from the v$datafile view to see if any of the datafiles are offline. If they are, bring them online, and then we can perform a recover database command.
l) Recovery of Read-Only tablespaces
If we have a current control file, then recovery of read only tablespaces is no different than recovering read-write files. The issues with read-only tablespaces arise if we have to use a backup control file. If the tablespace is in read-only mode, and hasn't changed to read-write since the last backup, then we will be able to media recovery using a backup control file by taking the tablespace offline. The reason here is that when we are using the backup control file, we must open the database with resetlogs. And we know that Oracle wont let us read files from before a resetlogs was done. However, there is an exception with read-only tablespaces. We will be able to take the datafiles online after we have opened the database.
When we have tablespaces that switch modes and we don't have a current control file, we should use a backup control file that recognizes the tablespace in read-write mode. If we don't have a backup control file, we can create a new one using the create controlfile command. Basically, the point here is that we should take a backup of the control file every time we switch a tablespaces mode.
