1. What is RMAN ?
Recovery Manager (RMAN) is a utility that can manage your entire Oracle backup and recovery activities.
Which Files must be backed up?
Database Files (with RMAN)
Control Files (with RMAN)
Offline Redolog Files (with RMAN)
INIT.ORA (manually)
Password Files (manually)
WHY USE RMAN ?
No extra costs …Its available free
?RMAN introduced in Oracle 8 it has become simpler with newer versions and easier than user managed backups
?Proper security
?You are 100% sure your database has been backed up.
?Its contains detail of the backups taken etc in its central repository
Facility for testing validity of backups also commands like crosscheck to check the status of backup.
Faster backups and restores compared to backups without RMAN
RMAN is the only backup tool which supports incremental backups.
Oracle 10g has got further optimized incremental backup which has resulted in improvement of performance during backup and recovery time
Parallel operations are supported
Better querying facility for knowing different details of backup
No extra redo generated when backup is taken..compared to online
backup without RMAN which results in saving of space in hard disk
RMAN an intelligent tool
Maintains repository of backup metadata
Remembers backup set location
Knows what need to backed up
Knows what is required for recovery
Knows what backups are redundant
UNDERSTANDING THE RMAN ARCHITECTURE
An oracle RMAN comprises of
RMAN EXECUTABLE This could be present and fired even through client side
TARGET DATABASE This is the database which needs to be backed up .
RECOVERY CATALOG Recovery catalog is optional otherwise backup details are stored in target database controlfile .
It is a repository of information queried and updated by Recovery Manager
It is a schema or user stored in Oracle database. One schema can support many databases
It contains information about physical schema of target database datafile and archive log ,backup sets and pieces Recovery catalog is a must in following scenarios
. In order to store scripts
. For tablespace point in time recovery
Media Management Software
Media Management software is a must if you are using RMAN for storing backup in tape drive directly.
Backups in RMAN
Oracle backups in RMAN are of the following type
RMAN complete backup OR RMAN incremental backup
These backups are of RMAN proprietary nature
IMAGE COPY
The advantage of uing Image copy is its not in RMAN proprietary format..
Backup Format
RMAN backup is not in oracle format but in RMAN format. Oracle backup comprises of backup sets and it consists of backup pieces. Backup sets are logical entity In oracle 9i it gets stored in a default location There are two type of backup sets Datafile backup sets, Archivelog backup sets One more important point of data file backup sets is it do not include empty blocks. A backup set would contain many backup pieces.
A single backup piece consists of physical files which are in RMAN proprietary format.
Example of taking backup using RMAN
Taking RMAN Backup
In non archive mode in dos prompt type
RMAN
You get the RMAN prompt
RMAN > Connect Target
Connect to target database : Magic
using target database controlfile instead of recovery catalog
Lets take a simple backup of database in non archive mode
shutdown immediate ; - - Shutdowns the database
startup mount
backup database ;- its start backing the database
alter database open;
We can fire the same command in archive log mode
And whole of datafiles will be backed
Backup database plus archivelog;
Restoring database
Restoring database has been made very simple in 9i .
It is just
Restore database..
RMAN has become intelligent to identify which datafiles has to be restored
and the location of backuped up file.
Oracle Enhancement for RMAN in 10 G
Flash Recovery Area
Right now the price of hard disk is falling. Many dba are taking oracle database backup inside the hard disk itself since it results in lesser mean time between recoverability.
The new parameter introduced is
DB_RECOVERY_FILE_DEST = /oracle/flash_recovery_area
By configuring the RMAN RETENTION POLICY the flash recovery area will automatically delete obsolete backups and archive logs that are no longer required based on that configuration Oracle has introduced new features in incremental backup
Change Tracking File
Oracle 10g has the facility to deliver faster incrementals with the implementation of changed tracking file feature.This will results in faster backups lesser space consumption and also reduces the time needed for daily backups
Incrementally Updated Backups
Oracle database 10g Incrementally Updates Backup features merges the image copy of a datafile with RMAN incremental backup. The resulting image copy is now updated with block changes captured by incremental backups.The merging of the image copy and incremental backup is initiated with RMAN recover command. This results in faster recovery.
Binary compression technique reduces backup space usage by 50-75%.
With the new DURATION option for the RMAN BACKUP command, DBAs can weigh backup performance against system service level requirements. By specifying a duration, RMAN will automatically calculate the appropriate backup rate; in addition, DBAs can optionally specify whether backups should minimize time or system load.
New Features in Oem to identify RMAN related backup like backup pieces, backup sets and image copy
Oracle 9i New features Persistent RMAN Configuration
A new configure command has been introduced in Oracle 9i , that lets you configure various features including automatic channels, parallelism ,backup options, etc.
These automatic allocations and options can be overridden by commands in a RMAN command file.
Controlfile Auto backups
Through this new feature RMAN will automatically perform a controlfile auto backup. after every backup or copy command.
Block Media Recovery
If we can restore a few blocks rather than an entire file we only need few blocks.
We even dont need to bring the data file offline.
Syntax for it as follows
Block Recover datafile 8 block 22;
Configure Backup Optimization
Prior to 9i whenever we backed up database using RMAN our backup also used take backup of read only table spaces which had already been backed up and also the same with archive log too.
Now with 9i backup optimization parameter we can prevent repeat backup of read only tablespace and archive log. The command for this is as follows Configure backup optimization on
Archive Log failover
If RMAN cannot read a block in an archived log from a destination. RMAN automatically attempts to read from an alternate location this is called as archive log failover
There are additional commands like
backup database not backed up since time '31-jan-2002 14:00:00'
Do not backup previously backed up files
(say a previous backup failed and you want to restart from where it left off).
Similar syntax is supported for restores
backup device sbt backup set all Copy a disk backup to tape
(backing up a backup
Additionally it supports
. Backup of server parameter file
. Parallel operation supported
. Extensive reporting available
. Scripting
. Duplex backup sets
. Corrupt block detection
. Backup archive logs
Pitfalls of using RMAN
Previous to version Oracle 9i backups were not that easy which means you had to allocate a channel compulsorily to take backup You had to give a run etc . The syntax was a bit complex …RMAN has now become very simple and easy to use..
If you changed the location of backup set it is compulsory for you to register it using RMAN or while you are trying to restore backup It resulted in hanging situations
There is no method to know whether during recovery database restore is going to fail because of missing archive log file.
Compulsory Media Management only if using tape backup
Incremental backups though used to consume less space used to be slower since it used to read the entire database to find the changed blocks and also They have difficult time streaming the tape device. .
Considerable improvement has been made in 10g to optimize the algorithm to handle changed block.
Observation
Introduced in Oracle 8 it has become more powerful and simpler with newer version of Oracle 9 and 10 g.
So if you really don't want to miss something critical please start using RMAN.
2. When you take a hot backup putting Tablespace in begin backup mode, Oracle records SCN # from header of a database file. What happens when you issue hot backup database in RMAN at block level backup? How does RMAN mark the record that the block has been backed up ? How does RMAN know what blocks were backed up so that it doesn't have to scan them again?
In 11g, there is Oracle Block Change Tracking feature. Once enabled; this new 10g feature records the modified since last backup and stores the log of it in a block change tracking file. During backups RMAN uses the log file to identify the specific blocks that must be backed up. This improves RMAN's performance as it does not have to scan whole datafiles to detect changed blocks.
Logging of changed blocks is performed by the CTRW process which is also responsible for writing data to the block change tracking file. RMAN uses SCNs on the block level and the archived redo logs to resolve any inconsistencies in the datafiles from a hot backup. What RMAN does not require is to put the tablespace in BACKUP mode, thus freezing the SCN in the header. Rather, RMAN keeps this information in either your control files or in the RMAN repository (i.e., Recovery Catalog).
3. What are the Architectural components of RMAN?
1.RMAN executable
2.Server processes
3.Channels
4.Target database
5.Recovery catalog database (optional)
6.Media management layer (optional)
7.Backups, backup sets, and backup pieces
4. What are Channels?
A channel is an RMAN server process started when there is a need to communicate with an I/O device, such as a disk or a tape. A channel is what reads and writes RMAN backup files. It is through the allocation of channels that you govern I/O characteristics such as:
Type of I/O device being read or written to, either a disk or an sbt_tape
Number of processes simultaneously accessing an I/O device
Maximum size of files created on I/O devices
Maximum rate at which database files are read
Maximum number of files open at a time
5. Why is the catalog optional?
Because RMAN manages backup and recovery operations, it requires a place to store necessary information about the database. RMAN always stores this information in the target database control file. You can also store RMAN metadata in a recovery catalog schema contained in a separate database. The recovery catalog
schema must be stored in a database other than the target database.
6. What does complete RMAN backup consist of ?
A backup of all or part of your database. This results from issuing an RMAN backup command. A backup consists of one or more backup sets.
7. What is a Backup set?
A logical grouping of backup files -- the backup pieces -- that are created when you issue an RMAN backup command. A backup set is RMAN's name for a collection of files associated with a backup. A backup set is composed of one or more backup pieces.
8. What is a Backup piece?
A physical binary file created by RMAN during a backup. Backup pieces are written to your backup medium, whether to disk or tape. They contain blocks from the target database's datafiles, archived redo log files, and control files. When RMAN constructs a backup piece from datafiles, there are a several rules that it follows:
A datafile cannot span backup sets
A datafile can span backup pieces as long as it stays within one backup set
Datafiles and control files can coexist in the same backup sets
Archived redo log files are never in the same backup set as datafiles or control files RMAN is the only tool that can operate on backup pieces. If you need to restore a file from an RMAN backup, you must use RMAN to do it. There's no way for you to manually reconstruct database files from the backup pieces. You must use RMAN to restore files from a backup piece.
9. What are the benefits of using RMAN?
1. Incremental backups that only copy data blocks that have changed since the last backup.
2. Tablespaces are not put in backup mode, thus there is noextra redo log generation during online backups.
3. Detection of corrupt blocks during backups.
4. Parallelization of I/O operations.
5. Automatic logging of all backup and recovery operations.
6. Built-in reporting and listing commands.
10) Why Online redo logs should never be backup or are never required to be backed up?
Online redo logs, unlike archived logs, should never be backed up. The chief danger associated by having backups of online redo logs is that you may accidentally restore those backups without meaning to, and corrupt your database.
Online redo log backups are also not particularly useful, for the following reasons:
■ If your database is in ARCHIVELOG mode, then the archiver is already archiving the filled redo logs automatically.
■ If your database is in NOARCHIVELOG mode, then the only type of physical backups that you can perform are closed, consistent, whole database backups. The files in this type of backup are all consistent and do not need recovery, so the online logs are not useful after a restore from backup.
The best method for protecting the online logs against media failure is to multiplex them
11) Can we possibly take backup of online logs by RMAN commands?
RMAN does not permit you to back up online redo logs. You must archive a redo log before backing it up.