Harvard University Oracle Database Administration Session 11 Database Recovery

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University The Change Sequence  Shutdown immediate  Startup mount  Alter database archivelog;  Alter database open;  Shutdown immediate  Take a full backup

Harvard University 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

Harvard University Recovery Process  Oracle performs recovery based on scn#  Two step Process – Roll Forward Redo files – Roll back Transaction recovery

Harvard University Failure Recovery  Media failure  User failure  Instance failure  Network failure  Process failure

Harvard University 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

Harvard University Database Startup Status  Nomount init.ora  Mount recover, alter database  Open database

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University Archivelog Mode Recovery  Place the database in either mount or open mode  Recover the data files by using the recover command

Harvard University 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

Harvard University 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

Harvard University Recover Syntax  Recover [automatic] database, can only be used for a closed database recovery  Recover [automatic] tablespace | | Can be used for open database recovery  Recover [automatic ] datafile | Can be used for both opened and closed database recovery

Harvard University 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!!

Harvard University 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;

Harvard University 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  Recover from ‘ ’ database;

Harvard University 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

Harvard University 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

Harvard University Recovery Process  V$archived_log gives a list of all archived log files

Harvard University 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’

Harvard University Scenario 1  Apply all needed archived and redo logs  Alter database open

Harvard University 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;

Harvard University 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;

Harvard University 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

Harvard University 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

Harvard University 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

Harvard University Incomplete Recovery  Recover [automatic] database until time ‘ :14:22:03’;  Recover database until time ‘ :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 ;

Harvard University 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

Harvard University 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

Harvard University 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.

Harvard University 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

Harvard University 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

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

Harvard University 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

Harvard University 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

Harvard University Data Pump Utility  Transfers data objects and data between Oracle databases  expdp help=y  Impdp help=y

Harvard University Data Pump Utility  Import options (modes) – Full – Schema (user) – Tablespace – Table  No mode, defaults to full

Harvard University 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)

Harvard University Data Pump Utility  Parameter file example – Directory= – Dumpfile=name.dmp – Content=metedata_only – Remap=joe:jane  The export and import function are desupported in 11g

Harvard University 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

Harvard University Reading  Chapter 11 DBA Handbook  Oracle Metalink  Backup and Recovery Handbook, Oracle Press  Backup and Recovery Concepts, Oracle Guide.