CERN/IT/DB

Oracle backup and recovery strategy

Catherine Delamare /IT DB

CERN/IT/DB

AGENDAWhy plan backups?

Overview of backups

Backup implementation in IT/DB

Export/Import

Recovery

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Why plan backups?

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Why plan backups?

Hardware never failsOracle never makes a mistakeUsers never make mistakesProgrammers never make mistakesPhysicists never make mistakesDatabase administrators (DBAs)

never make mistakes

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Why plan backups?

The only insurance you have against failures causing loss of data:

• Human errors• Hardware failures• Power failures• Software errors

DBA’s job security!

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Why plan backups?

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Overview of backups

Physical backups protect against global failure Cold backups : database needs to be

stopped Hot backups : database keeps on running

Logical backups (or “exports”) protect against deletion of (a) table(s) can be run by the user himself

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ARCHIVELOG Mode or not? (1/3)

Definition All changes are kept in “archived logs” The more activity, the more archived logs Central db: 600 logs per week (30M each=18G

transactions volume per week)

Advantages of ARCHIVELOG mode Complete recovery up to the last committed

transaction before the failure (“point-in-time” recovery)

Mandatory for mission-critical applications Pre-requisite to run hot backups

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ARCHIVELOG Mode or not? (2/3)

Sunday Friday

BACKUP CRASH

54321

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ARCHIVELOG Mode or not? (3/3)

ARCHIVELOG mode implications: Additional overhead

(even more overhead during online backup) Oracle hangs if the archived disk fills up Archived logs must be kept in a safe place

• Tape facilities implied

More work for the DBA: • Space management• Log files tracking

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Offline backup (COLD)

Database must be down

Backup of all datafiles, redo log files and control files

Disk space needed

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Benefits of offline backup

Best guarantee for a restartable database (Oracle recommends a weekly cold backup)

Quick recovery (by replacing all datafiles, redo log files and control files by their backup version)

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Drawbacks of offline backup

Database must be downLoss of data between backup and

failure if not using ARCHIVELOG mode

Corruption may not be recognized!Might not backup everything:

Backup scripts must be often checked and tested!

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Online (hot) backup

Meaningful only if database is operating in ARCHIVELOG mode: if not, hot backups are useless for recovery

Database must be up and open Do a favor to users, schedule it during off

hoursThe unit of an online backup is a

tablespace

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Benefits of online (hot) backup

Database remains available

Control over what to backup and when

Not all tablespaces need to be backed up at the same time

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Drawbacks of online (hot) backup

Setup and administration is more complex

Leading cause of recovery problems

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Online backup procedure

ALTER TABLESPACE tsp BEGIN BACKUPCopy/Backup all datafiles in the

tablespaceALTER TABLESPACE tsp END BACKUPALTER SYSTEM SWITCH LOGFILEBackup archived log filesALTER DATABASE BACKUP

CONTROLFILE to ‘filespec’

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Backups implementation (1/3)

Most production databases run in ARCHIVELOG mode

Archived redo logs stored on two different tape devices

Home-grown scripts perform depending on the service:

• daily/weekly cold/hot backup +• daily full export

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Backups implementation (2/3)

On mission-critical systems : Shutdown the oracle database Detach the mirrors Restart the database on one side => service

interruption of the order of 1-6 minutes Backup the frozen side Reattach the mirrors

E.g.:central cluster, cryogenics, remedy,tape management and radio-frequency databases

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Backups implementation (3/3)

On the central database cluster..The 2 nodes synchronize a clean

rundown of the common database• Node A detaches the database mirrors

and restarts DB • Tells node B to restart as well• Performs a cold backup onto disks• Reattaches the mirrors

Service interruption: 6 minutes

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Oracle EXPORT/IMPORT

The Export and Import Oracle utilities generate a file with a logical copy of the data and application

Export and Import are useful to recover specific items lost due to user errors

Export and Import help migration to different releases of the Oracle RDBMS

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EXPORT/IMPORT(cont’d)

Export and Import support object typesExport writes object type definitions and

all associated data to the dump fileImport then re-creates these items from

the dump fileThe definition statements for derived

types are exported, same applies to OIDs

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EXPORT (logical)

Database must be up and runningExport reads the database using SQL

Export file contains create and insert statements

Export provides a Read consistent view of the database Changes made after export begins are

not included

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Benefits of export

Easy to recover individual itemsPortable:

can be used to move data from one machine to another

useful in a heterogeneous environmentUnlike backup, export identifies

physical data block corruption since it performs a full table scan while exporting a table

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Drawbacks of export

Does not provide point in time recovery

Export/Import are usually long processes.To improve speed: import with ROWS=Y INDEX=N to

restore data then import with ROWS=N and INDEX=Y

to build indexes

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Export/import examplesExport of a full database (from a DBA):

EXP FULL=Y FILE=fullexp.dmp LOG=fullexp.log

Export of a specific schema:EXP USER/PASSWORD TABLES=(RUN1,RUN2) FILE=runs12.dmpLOG=runs12.log

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Recovery considerations

Media recovery: Done in response to a RECOVERY

command Has to be done by a DBA after

identifying the appropriate recovery action depending on the failure (datafile, tablespace, database)

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Recovery in NOARCHIVELOG mode

Only offline backup (or export) can be used

What to do: Restore all copies of datafiles, control

files and redo log files startup the database

You are back in the status when the backup (export) were run

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Recovery in ARCHIVELOG mode

Only way to perform the point in time recovery

What to do: Restore from backup the damaged

Oracle files Apply Redo log changes

• Database recovery• Tablespace recovery• Datafile recovery

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Benefits of recovery

Database recovery can be complete or not: recover database (complete) recover database until time ‘2001-07-

10:10:05:00’ recover database until cancel recovery of TABLESPACE or DATAFILE

can only be complete

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Implications of recovery

Database is not open to the usersIt can take a long time depending

on the number of redo log files to be applied.

Must have ALL required files (datafiles, archived redo log files).

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SUMMARY

5 real cases of Oracle recovery have been performed in the past 7 years (Successful…)

Many items have been recovered from export files to repair accidental deletion of items

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