Part II Part III
Oracle Backup and Recovery on Windows, Part I - Database Backups using RMAN:
(Revised: 24 Jan 2005)
Note: The scripts listed below should be customised and tested thoroughly before implementation on a production system. Please read through the Oracle backup documentation before implementing your backups. References have been listed at the end of this article.
This article is the first in a series of three, introducing Oracle's RMAN (Recovery MANager) backup and recovery tool to novice DBAs. The articles focus on the Windows operating system, but can be easily adapted to other platforms. RMAN will work in the same manner on all operating systems (OS) that Oracle runs on. However, OS functions such as script scheduling etc. obviously vary from platform to platform. The present article describes the implementation of a simple Oracle backup strategy using RMAN. The next article will discuss some recovery scenarios within this backup framework, and the final article will deal with disaster recovery.
We begin with a statement of assumptions regarding the database environment and business requirements regarding data recoverability. This serves to anchor the subsequent discussion in a definite context. We then briefly discuss the files relevant to operation of Oracle. Finally, we move on to the main topic - a discussion of RMAN and how it can be used to backup a database.
Following are the assumptions pertaining to the database environment and business expectations regarding data recovery:
2. Oracle database files:
In order to perform backups effectively, it is necessary to understand a bit about the various files that comprise a database. This section describes the files that make up an Oracle database. The descriptions given here are very brief. Please check the Oracle Administrator's Guide for further details.
Oracle requires the following files for its operation:
3. OS backup utilities vs. RMAN- a brief discussion:
OS Backup utilities copy OS files from disk to tape. By themselves they are not useful for database backups, unless the database is closed. The reason they cannot be used to backup open databases is as follows: If the database is open, it is possible that contents of a datafile block are being modified at the very instant that the block is being copied by the utility. In such a situation the copy of the block would be inconsistent, and hence useless for recovery. The way to avoid this is to put a tablespace into a special "backup mode" before copying the datafiles associated with the tablespace. Such OS level backups, also called user managed backups, are the traditional (non-RMAN) way to backup Oracle databases. When a tablespace is in backup mode, the SCN, which is marked in the header of each datafile in the tablespace, is frozen until the tablespace is taken out of backup mode. Additionally, whenever a data block in the tablespace is modified, the entire block is copied to the online redo log (in contrast to only modified rows being copied when the tablespace is not in backup mode). This causes a huge increase in the redo generated, which is a major disadvantage of user managed backups.
One can perform user managed backups of a database using homegrown scripts. Such a script would cycle through all tablespaces in the database, putting each tablespace in backup mode, copying the associated datafiles and finally switching the tablespace out of backup mode. A fragment of a user managed hot backup script for Windows might read as follows:
--put USERS tablespace in backup modeThe above would be invoked from sqlplus, via an appropriately scripted batch file.
alter tablespace users begin backup;
--copy files belonging to USERS tablespace
host copy d:\oracle\ora92\orcl\users.dbf e:\backup;
--take USERS tablespace out of backup mode
alter tablespace users end backup;
--continue with other tablespaces and then copy other oracle files...
Most OS backup utility vendors provide optional add-ons that automate the process of user managed backups. These add-ons, which usually do no more than the script shown above, are sold as extra cost add-ons to the base backup software.
RMAN is a database backup utility that comes with the Oracle database, at no extra cost. As such, it is aware of the internal structure of Oracle datafiles and controlfiles, and knows how to take consistent copies of data blocks even as they are being written to. Furthermore, it does this without putting tablespaces in backup mode. Therefore RMAN does not cause a massive increase in generated redo. Another advantage of using RMAN is that it backs up only those blocks that have held or currently hold data. Hence RMAN backups of datafiles are generally smaller than the datafiles themselves. In contrast, OS copies of datafiles have the same size as the original datafiles. Finally, with RMAN backups it is possible to recover individual blocks in case of block corruption of datafiles. Considering the above, it makes good sense to use RMAN instead of vendor supplied add-ons or homegrown user managed backup scripts.
4. Configuring RMAN:
RMAN is a command line utility that is installed as a part of a standard database installation. Note that RMAN is only a command interface to the database - the actual backup is performed by a dedicated server process on the Oracle database.
RMAN can invoked from the command line on the database host machine like so:
C:\>rman target /The first line is the one we type and the remaining ones are feedback from execution of the command. The net result is to leave us connected to the target database - the database we want to back up - with the RMAN> prompt, indicating that RMAN is ready for further work. Here we have invoked RMAN on the server, and have logged on to the server using an account that belongs to the ORA_DBA OS group. As described earlier, this enables us to connect to the target database (as sysdba - this is implicit) without having to supply a password. Note that on Windows, one must also set SQLNET.AUTHENTICATION_SERVICES=(NTS) in the sqlnet.ora file order to connect using OS authentication as above.
Recovery Manager: Release 184.108.40.206.0 - Production
Copyright (c) 1995, 2002, Oracle Corporation. All rights reserved.
connected to target database: ORCL (DBID=1036216947)
At this point a digression is in order. RMAN can be run in two modes - catalog and nocatalog. In the former, backup information and RMAN scripts are stored in another database known as the RMAN catalog. In the latter, RMAN stores backup information in the target database controlfile. Catalog mode is more flexible, but requires the maintenance of a separate database on another machine (there's no point in creating the RMAN catalog on the database to be backed up!). Nocatalog mode has the advantage of not needing a separate database, but places more responsibility on the controlfile. We will use nocatalog mode in our discussion, as this is a perfectly valid choice for sites with a small number of databases.
RMAN can be configured through various persistent parameters. Note that persistent parameters can be configured only for Oracle versions 9i and better. The current configuration can be seen via the "show all" command:
RMAN> show all;
RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 2;
CONFIGURE BACKUP OPTIMIZATION OFF; # default
CONFIGURE DEFAULT DEVICE TYPE TO DISK; # default
CONFIGURE CONTROLFILE AUTOBACKUP ON;
CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO 'e:\backup\ctl_sp_bak_%F';
CONFIGURE DEVICE TYPE DISK PARALLELISM 2;
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT 'e:\backup\%U.bak' MAXPIECESIZE 4G;
CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT 'e:\backup\%U.bak' MAXPIECESIZE 4G;
CONFIGURE MAXSETSIZE TO UNLIMITED; # default
CONFIGURE SNAPSHOT CONTROLFILE NAME TO 'C:\ORACLE\ORA92\DATABASE\SNCFORCL.ORA'; # default
The reader is referred to the RMAN documentation for a detailed explanation of the options attached to each of these parameters. Here we will discuss only those of relevance to our backup requirements.
RMAN> configure controlfile autobackup off;5. Scripting the backup:
using target database controlfile instead of recovery catalog
old RMAN configuration parameters:
CONFIGURE CONTROLFILE AUTOBACKUP ON;
new RMAN configuration parameters:
CONFIGURE CONTROLFILE AUTOBACKUP OFF;
new RMAN configuration parameters are successfully stored
With the background stuff out of the way, we now move on to the actual backup. We will write a simple script that will backup our database, verify that the backup can be restored and then delete all obsolete backups and archive logs (based on a redundancy of 2, as discussed above). The Windows scheduler will be used to run the script at a time of our choosing.
An Aside: Before we move on it is worth stating that RMAN can perform full or incremental backups. Full backups, as their name suggests, are backups of every data block in the datafiles. In contrast, Incremental backups backup only those database blocks that have changed since the last higher level backup. It would take us too far afield to detail the intricacies of incremental backups - we refer you to the Oracle documentation for more details on this. For the case at hand, we can afford to perform full backups every night as the database is relatively small.
The backup script, which we store in a file named "rman_backup.rcv", is very simple:
#contents of rman_backup.rcv. "#" denotes a comment line, and will be ignored by RMAN.
backup database plus archivelog;
restore database validate;
delete noprompt obsolete;
host 'copy C:\oracle\ora92\database\pwdorcl.ora e:\backup';
host 'copy C:\oracle\ora92\network\admin\tnsnames.ora e:\backup';
host 'copy C:\oracle\ora92\network\admin\listener.ora e:\backup';
The script backs up the database and all archive logs and then checks that the backup can be restored. After that it deletes backups according to the configured retention policy - the "noprompt" in the delete command instructs RMAN not to prompt us before deleting files. Finally it does an OS copy of the password file and the relevant network configuration files. The RMAN "host" command enables us to execute any operating system command (on Linux, for instance, we would use "cp" instead of "copy"). In the above script the database name is ORCL, hence the password file is pwdORCL.ora. You will need to adapt each of the "host 'copy..." commands in the script to your specific directory structure and filenames. As an aside, it is worth pointing out that SQL commands can be executed from RMAN. A couple of examples:
sql 'alter system archive log current';
sql "create pfile=''e:\backup\initORCL.ora'' from spfile";
The "sql" keyword tells RMAN what follows is to be interpreted as an SQL command. The actual SQL should be enclosed in single or double quotes. The latter is useful if the command contains single quotes, as in the second example above. Note: In the second example, the quotes enclosing the pfile path are two single quotes, and the quotes enclosing the entire command are double quotes.
The script, rman_backup.rcv, is invoked by the following one line batch file:
REM contents of rman_backup.bat
rman target / cmdfile rman_backup.rcv log rman_backup.log
The "target /" indicates that the script logs on to Oracle as sysdba via an OS account that belongs to the ORA_DBA group. The "cmdfile" option indicates the name of the command file that RMAN should execute, in this case it is rman_backup.rcv. The "log" option tells rman that we want a transcript of the RMAN session to be stored in the file that follows the option - rman_backup.log in this case. Remember to check the log file once between each backup for any errors that may have occurred. The log file is overwritten on each execution of the batch file so it may be worth changing the name to include a unique identifier (such as a timestamp). The backup scripts could reside anywhere on the server, but it may be best to keep them in e:\backup so that they are archived off to tape along with the backups.
The next step is to schedule our batch file (rman_backup.bat) to run at the desired interval. This is done by scheduling the batch file via the Window Scheduler wizard, which is accessed through Control Panel>Scheduled Tasks>Add Scheduled Task>.
Finally, it should be ensured that the entire backup directory (e:\backup) is copied to tape nightly, after the database backup has been completed. There is no need to backup any other Oracle related directory. The tapes must be stored offsite so that they aren't destroyed in case the site is struck by disaster. In a disaster situation, we can recreate the database and then restore and recover data files (with up to a 24 hour data loss), using the backups that are on tape. The procedure for recovering from a disaster will be covered in the third article of this series. In case the database fails (say due to datafile corruption, for example) but the host server remains available, we can recover right up to the instant of failure using the backup on disk together with all archive logs since the backup and the current online redo logs. Some of these scenarios will be covered in the next article of this series.
6. Summary and Further Reading:
This article provides steps on setting up automated RMAN based backups of Oracle databases on Windows. As with all critical DBA tasks, scripts and commands described above should be customised to your requirements and tested thoroughly before implementation on your production systems.
In the interest of brevity, we have had to rush through some of the detail that is relevant to backup and recovery. The reader is therefore urged to read the pertinent Oracle documentation for complete coverage. The books of interest are:
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