Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

5.5 Backup and Recovery

This
section
provides only a very brief overview of standard backup and recovery
options. For more detailed information about backup and recovery
options, refer to Chapter 10.

Even if you've taken adequate precautions, critical
database records can sometimes be destroyed as a result of user error
or hardware or software failure. The only way to prepare for this
type of potentially disastrous situation is to perform regular backup
operations.

Two basic types of potential failures can affect an Oracle
database: instance failure,
in which the Oracle instance terminates without going through the
shutdown process; and media failure, in
which the disks that store the information in an Oracle database are
corrupted or damaged.

After an instance failure, Oracle will
automatically perform crash recovery; you can use Real
Application Clusters (Oracle Parallel Server prior to
Oracle9i) to automatically perform instance
recovery when one of its instances crashes.
However, DBAs must
initiate recovery from media failure. The ability to recover
successfully from this type of failure is one of the greatest
challenges a DBA facesit's also the place
where the value of the DBA becomes most apparent! The recovery
process includes restoring older copies of the damaged datafile(s)
and rolling forward by applying archived and online redo logs.

To ensure successful
recovery, the DBA should have prepared for this eventuality by
performing the following actions:

Multiplexing online redo logs by having multiple log members per
group on different disks and controllers

Running the database in ARCHIVELOG mode so that redo log files are
archived before they are reused

Archiving redo logs to multiple locations

Maintaining multiple copies of the control file(s)

Backing up physical datafiles frequentlyideally, storing
multiple copies in multiple locations

Running the database in ARCHIVELOG mode ensures that you can
recover the database up to the time of the media failure; in this
mode, the DBA can perform online datafile backups while the
database is available for use. In addition, archived redo logs can be
sent to a standby database (explained in Chapter 10) in which they
may be applied.

RMAN, first introduced in Oracle8 and greatly enhanced since,
provides an easy-to-use frontend to manage this process. RMAN is
accessible today through EM interfaces.

5.5.1 Types of Backup and Recovery Options

There are two major categories of backup:

Full backup

Includes
backups of datafiles, datafile copies, tablespaces, control files
(current or backup), or the entire database (including all datafiles
and the current control file). Reads the entire file and copies all
blocks into the backup set, skipping only datafile blocks that have
never been used (with the exception of control files and redo logs
where no blocks are skipped).

Incremental backup

Includes backups of datafiles, tablespaces, or the whole database.
Reads the entire file and backs up only those data blocks that have
changed since a previous backup.

You can begin backups through the Recovery Manager (RMAN) or the
Oracle Enterprise
Manager interface to RMAN, which uses the database export facility,
or you can initiate backups via standard operating system backup
utilities.

RMAN was introduced with Oracle8 and replaced the Enterprise Backup
Utility available for some previous Oracle7 releases. In general,
RMAN supports the most database backup features, including open or
online backups, closed database backups, incremental backups at the
Oracle block level, corrupt block detection, automatic backups,
backup catalogs, and backups to sequential media.
RMAN added capabilities in

Oracle9i for
one-time backup configuration, recovery windows to determine and
manage expiration dates of backups, and restartable backups and
restores. Also added was support for testing of restores and
recovery.

In Oracle Database
10g, RMAN can perform image copy backups of the
database, tablespaces, or datafiles. RMAN can be used to apply
incremental backups to datafile image backups. The speed of
incremental backups is increased through a change tracking feature by
reading and backing up only changed blocks.

Recovery
options include the following:

Complete database recovery to the point of failure

Tablespace point-in-time recovery (recovery of a tablespace to a time
different from the rest of the database)

Time-based or point-in-time database recovery (recovery of the entire
database to a time before the most current time)

Recovery until the CANCEL command is issued

Change-based or log sequence recovery (to a specified System Change
Number, or SCN)

You can recover through the use of RMAN (utilizing the recovery
catalog or control file) or via SQL or SQL*Plus.

RMAN in Oracle Database 10g improves the reliability of backups and
restores through a number of added features. The current database
supports the backup and restore of standby control files. RMAN now
automatically retries a failed backup or restore operation. During
recovery, RMAN automatically creates and recovers datafiles that have
never been backed up. Where backups are missing or corrupt during the
restore process, RMAN now automatically uses an older backup.

To speed backups and restoration, Oracle Database 10g introduces the Flash
Recovery Area, organizing recovery files to a specific area on disk.
These files include control files, archived log files, flashback
database logs, datafile copies, and RMAN backups. You can set a
RETENTION AREA parameter to retain
needed recovery files for specific time windows. Backup files and
archivelogs that age beyond the time window are automatically
deleted. Oracle Database 10g's ASM (described
earlier in this chapter) can configure the Flash Recovery Area. If
availability of disk space is an issue, RMAN in Oracle Database 10g
also has the ability to compress backup sets.

5.5.2 Additional Backup Capabilities

A number of Oracle
Backup Solutions
Program (BSP) partners certify their products to perform backup and
recovery to disk and tape storage devices using RMAN. Oracle bundles
Legato's
Single Server Version (LSSV) tape storage management product at no
charge on popular Unix platforms (such as AIX, HP-UX, Linux, Solaris,
and Tru64 Unix) and Windows platforms. The LSSV's
capabilities include the following:

Media management, including tape labeling, media tracking, and
retention policy management

Ability to use up to four locally connected tape drives and up to
four concurrent data streams

Installation integrated into the Oracle installer

Integration with EM for administration

There are some limitations when using LSSV. The product
doesn't provide networked backups; it provides
support for only a limited number of tape devices; and it
doesn't support robotic libraries or backups of
operating system files, network attached storage (NAS), or storage
area networks (SAN). Legato offers optional products providing these
additional backup capabilities.

There are other Oracle BSP partners, such as Computer Associates,
EMC, Hewlett-Packard, Sun, IBM Tivoli, and Veritas. Current partners
are generally posted at http://otn.oracle.com (the Oracle Technology
Network). You may want to check with your favorite backup vendor
about their current certification in support of
Oracle's Media Management Interface Library (MML),
the interface to RMAN.