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Oracle Database Administration

Session 11

Database Recovery

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ArchiveLog Mode

Log_archive_dest = filename– Filename is /disk1/archive/arch

Log_archive_dest should not be on a raw device

Log_archive_format = extension– Extension includes %s, so that the log sequence

number is part of the filename

Log_archive_format = %s.arc

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ArchiveLog Mode

The file will be named arch%s.arc Example /disk1/archive/arch23005.arc We can duplex the archive log files Log_archive_duplex_dest = filename

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ArchiveLog Mode

After changing a database from noarchivelog mode to archivelog mode, take a backup of the database files and the control file

The previous backups are now unusable

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The Change Sequence

Shutdown immediate Startup mount Alter database archivelog; Alter database open; Shutdown immediate Take a full backup

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The Change Sequence

In the init.ora set the log_archive_start = true, so that the database will startup in archivelog mode, automatically

You can set log_archive_max_processes = N if you want multiple archive processes, ARCn

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Recovery Process

Oracle performs recovery based on scn# Two step Process

– Roll Forward Redo files– Roll back Transaction recovery

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Failure Recovery

Media failure User failure Instance failure Network failure Process failure

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Failure Recovery

Each redo file has a low and high SCN select * from v$log_history Log switches

– Alter system switch logfile

Checkpoints– Flushes modified data from cache to disk

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Database Startup Status

Nomount init.ora Mount recover, alter database Open database

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Noarchivelog Mode Recovery

When media failure occurs, a valid closed database backup must exist in order to recover

All Oracle files must be restored , even if only one data file is damaged or lost

Restore all data files, control files and redo logs

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Noarchivelog Mode Recovery To recover a database that is in noarchivelog

mode Shutdown abort Delete existing files Recover all the files Restart the instance Inform users they need to re-enter data from

the time of the last backup

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Recovery to a Different Location

To restore file(s) to a different location, if the original disk is unusable

– Copy the file(s) to the new location Mount the instance, use ‘startup mount’ alter database rename file

‘/u04/oradata/user_01.dbf’ to ‘/u05/oradata/user_01.dbf’;

This command updates the control file with the new file location

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Recovery to a Different Location

Make sure that there is a copy of the file in the new location before issuing the command

Open the database

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Archivelog Mode Recovery

To recover a database that is in archivelog mode, we need

– A valid backup containing the lost or damaged data files, after the database was set in archivelog mode

– All archived logs from the backup you are restoring from, to the present

– The redo logs that contain the transactions that are not yet archived

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Archivelog Mode Recovery

If we meet all the above requirements Make sure that the file(s) that are to be

restored are not open Check v$datafiles and v$tablespace to get the

status Restore only the lost or damaged file(s) Do not restore online redo logs

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Archivelog Mode Recovery

Place the database in either mount or open mode

Recover the data files by using the recover command

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Archivelog Mode Recovery

The advantages of the archivelog mode – Only need to restore the lost or damaged file(s)– No committed data is lost– Restoring the files and then applying the archived

and redo logs, takes the database up to the current point-in-time

– The restore time is reduced

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Archivelog Mode Recovery

The disadvantages of the archivelog mode– We need all the archived logs from the time of

the last backup to the current time– If one is missing or damaged, we cannot

complete recovery, because the archives are applied in sequence

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Recover Syntax

Recover [automatic] database, can only be used for a closed database recovery

Recover [automatic] tablespace <number>| |<name> Can be used for open database recovery

Recover [automatic ] datafile <number> | <name> Can be used for both opened and closed database recovery

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Recover Syntax

Automatic means that the archives and redo logs are applied automatically

Alter database can be placed in front of the recover command but will suppress some of the error messages. Do not use!!

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Recover Syntax

Recover database; Recover datafile ‘/u04/oradata/data_01.dbf’; Alter database recover database; Recover tablespace user_data; Recover datafile 2; Alter database recover datafile 2;

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Recover Syntax

If we restore the archived logs from a different location than log_archive_dest, we must tell the database where they are

Alter system archive log start to < new location, full path>

Recover from ‘<new location, full path>’ database;

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Recovery Process

When we issue recover datafile 4;, oracle will suggest(request) a specific log to apply.

If it exists in the log_archive_dest , enter auto and it will be applied

If you issue recover automatic datafile 4; It will apply the log , if it exists

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Recovery Process

To find the files needing recovery – select * from v$recover_file;

The change# column returns the SCN (system change number) from where recovery must start

To get a list of all the archived log files needed for recovery, look in v$recovery_log

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Recovery Process

V$archived_log gives a list of all archived log files

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Scenario 1 Corrupt data blocks on /u02, in a data file

belonging to the SYSTEM tablespace Shutdown the database Restore the damaged file from the most

recent valid backup Startup mount Recover datafile ‘/u02/oradata/system01.dbf’

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Scenario 1

Apply all needed archived and redo logs Alter database open

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Scenario 2

A datafile is accidentally removed using unix commands

See what tablespace it belongs to Select file_id f#, file_name, tablespace_name

tablespace, status from dba_data_files;

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File Offline Do we need to take the file offline Select d.file# f#, d.name, d.status, h.status

from v$datafile d, v$datafile_header h where d.file# = h.file#;

If the file is offline , restore it Recover datafile ‘/u04/oradata/data02.dbf’; Or use recover tablespace user_data;

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File Not Offline If the file is not offline Alter database datafile

‘/u04/oradata/data02.dbf’ offline After all recovery is complete, alter the datafile

online If a tablespace is taken offline, then all the

datafiles related to that tablespace are offline

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Incomplete Recovery

Reasons– User error, like table dropped or bad data

committed

Recovery fails because an archive log is lost Loss of all control files Loss of all unarchived redo log files and a

data file

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Incomplete Recovery

Types of incomplete recovery– Cancel-based recovery– Time-based recovery– Recovery using a backup control file

recover database until cancel; Used when redo log file or group is damaged

or when needed archive log file is lost

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Incomplete Recovery

Recover [automatic] database until time ‘2000-04-12:14:22:03’;

Recover database until time ‘2000-04-12:14:22:03’ using backup controlfile;

Time is ‘YYYY-MM-DD:HH:MI:SS’; The application of the archived redo logs can

be automatic Recover database until scn <integer>;

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Incomplete Recovery

Incomplete recovery can only occur if the database is in archivelog mode

We need a valid offline or online backup of all data files

All archived logs, from the restored backup to a time before the failure

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Incomplete Recovery

Perform a full offline backup of the existing database, including control files and redo logs

Restore all data files to take the database back in time

Place the database in mount mode and recover the database

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Incomplete Recovery

Open the database using the resetlogs option– Alter database open resetlogs;– This prevents the application of redo logs that

were skipped

Most recovery errors are due to the DBA.

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Import The import utility reads the export dump file

and runs the commands that are stored there It can be used to selectively bring back

objects or users from the export dump file If the export is incremental, then import from

the incremental dump file first, then from the most recent complete export

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Import

Some of the import parameters conflict with one another, eg FULL=Y and OWNER=GL

DESTROY=N is useful if we have two databases on a server and wish to export from one to another. The tablespace definitions are in the file and it will overwrite the existing files

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Import

Another way is to recreate tablespaces first, on the second database. This will avoid the overwrite

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Import

To import into another account use the following example

Exp system/passwd file=user1.dmp owner=joe grants=N indexes=Y compress=Y rows=Y

Imp system/passwd file=user1.dmp fromuser=joe touser=jane rows=Y indexes=Y

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Import

Create a parameter file that hold all the parameters for an export or an import

List each one on a new line of a file Use exp parfile=exp_parm_file Userid=system/passwd

Tables=(emp,dept)

File=export.dmp

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Data Pump Utility

Transfers data objects and data between Oracle databases

expdp help=y Impdp help=y

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Data Pump Utility

Import options (modes)– Full– Schema (user)– Tablespace– Table

No mode, defaults to full

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Data Pump Utility

To import the directory must be already setup for that database

The dump file must be in that directory Example

– Impdp user/pw parfile=name.par– A parameter file hold the various parameters to

complete the import ( or export)

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Data Pump Utility

Parameter file example– Directory= <location>– Dumpfile=name.dmp– Content=metedata_only– Remap=joe:jane

The export and import function are desupported in 11g

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Failure analysis Mean time between failures Mean time to recover when a failure occurs Downtime

– Scheduled outage for regular preventative maintenance

– Unscheduled outage due to physical failure

Record why failure occurred Service level agreements

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Reading

Chapter 11 DBA Handbook Oracle Metalink Backup and Recovery Handbook, Oracle

Press Backup and Recovery Concepts, Oracle

Guide.