Database BACKUP & recovery

DATABASE BACKUP & RECOVERY

Why Backup Data? Backing up data is vital for businesses

Lost information can cause major crisis or lead to business failure

Common ProblemsSystem outage

○ Power and hardware failureTransaction failure

○ Users may inadvertently corrupt the databaseMedia Failure

○ Disk drive becomes unusableDisaster

○ Database facility damaged by fire, flooding or other catastrophe

Database Recovery Mechanism for restoring a database quickly

and accurately after loss or damage RESPONSIBILITY OF ????? Recovery facilities:

• Backup Facilities• Journalizing Facilities• Checkpoint Facility• Recovery Manager

Back-up Facilities A DBMS COPY utility that produces a

backup copy (save) of the entire database or a subset of the database

Backup: user data, catalogs and all control files (buffer pool files, table space file, database configuration file)

Periodic backup (e.g. nightly, weekly) Backups stored in secure, off-site location Backup copy-used to restore the database

Back-up Facilities Cold backup / offline backup (DB2)–

database is shut down during backup Hot backup / online backup (DB2) –

selected portion is shut down and backed up at a given time

Incremental backups: record changes made since the last backup

Differential backups: record changes made since the last full/normal backup the differences since the last full backup.

Database backup syntax (DB2)BACKUP DATABASE <dbname>[ONLINE] [TO <dest_path>]

Offline backup exampleBACKUP DATABASE sample TO C:\backups Online backup exampleBACKUP DATABASE sample ONLINE TO C:\backups

Database backups – File naming convention

Table space backup Enable user to backup a subset of database Multiple table spaces can be specified Database must be using archival logging Table space backup can run in both online

and offline backup Table space can be restored from either

database backup or table space backup Use keyword TABLESPACE to specify table

spaces

Database Backup In DB2, backup can now be

automatically done and/or compressed To configure automatic backup:

Graphical user interface toolCommand line interfaceStored procedure

Back-up Facilities Database downtime can be very

expensive The lost revenue needs to be balanced

against the cost of additional technology, primarily disk storage, to achieve a desired level of availability

To achieve: some DBMS automatically make backup copies in real time.

Stored in on separate disk drives

Journalizing Facilities Keep track of changes made to database objects

and their data using audit trail / log In the event of failure: consistent database state

can be reestablished using the information in the journals together with the most recent complete backup

Can be stored in files or raw devices Logging is always ON for regular tables in DB2• It's possible to mark some tables or columns as NOT LOGGED• It's possible to declare and use USER temporary tables which may not be logged

Database logging

Types of logs Types of logs – based on log file allocation:

Primary logs are PRE-ALLOCATED Secondary logs are ALLOCATED as needed (costly)

For day to day operations, ensure that you stay within your primary log allocation

Types of logs – based on information stored in logs: Active logs:

○ Information has not been externalized (Not on the tablespace disk) Archive logs

○ All information externalized

Types of logging Circular logging

For non-production systemsLogs that become archived, can be overwrittenWhat if information externalized to the table space

was wrong? (Human error). No logs to redo things!

Archival loggingFor Production systemsNo logs are deleted.

○ Some are stored online (with active logs), others offline in an external media

Checkpoint Facilities A facility by which the DBMS periodically refuses to accept new

transactions. The system is in a quiet state and the database and transaction logs are synchronized

All transactions in progress are completed and journal files are brought up-to-date

DBMS writes a special record (checkpoint record) to the log file: snapshot of the state of the database

Checkpoint record contains information necessary to restart the system

Any dirty data blocks (pages of memory that contain changes that have not yet been written out to disk) are written from memory to disk storage

Automatically or response to commands in user application programs

This allows recovery manager to resume processing from short period, instead of repeating entire day

Recovery Manager A module of the DBMS that restores the

database to a correct condition when a failure occurs and then resumes processing user requests.

Type of restart used depends on the nature of failure.

Recovery and Restart Procedures

Disk Mirroring–switch between identical copies of databases

Restore/Rerun–reprocess transactions against the backup

Transaction Integrity–commit or abort all transaction changes

Backward Recovery (Rollback)–apply before images

Forward Recovery (Roll Forward)–apply after images (preferable to restore/rerun)

Disk Mirroring


Disk Mirroring Database must be mirrored switch to an

existing copy of the database 2 copies of the database must be kept &

updated simultaneously Media failure occurs: processing switch to

the duplicate copy Allows fastest recovery Hot-swappable: damaged disk can be

rebuilt from mirrored disk with no disruption in service to user


Restore/Rerun Involves reprocessing the day’s

transactions (up to the point of failure) against the backup copy of the databaseDatabase is shut downThe most recent copy of the database /file

to be recovered is mountedAll transactions that have occurred since

that copy (stored on the transaction log) are rerun


Restore/Rerun Advantage:

Simplicity DBMS does not need to create a database change

journal & no special restart procedures required Disadvantages:

Time to reprocess transactions may be prohibitive Processing of new transactions delayed until recovery completed

Sequencing of transactions will often be different from when they were originally processed: may lead to different results. Original Run: customer deposit may be posted before withdrawal Rerun: Withdrawal transaction may be attempted first.

Last resort in database processingRecovery and Restart Procedures

DB2 Restore Utility Restore utility is the complement of backup

utility Restores database or table space from a

previously taken backup TAKEN AT – specify the time stamp of the

database backup image. Backup image timestamp is displayed after successful completion of a backup

Without prompting – overrides any warnings

Example DB2 RestoreSAMPLE.0.DB2INST.NODE0000.CATN0000.20110718131210.001

RESTORE DATABASE dbalias FROM <db_path> TAKEN AT 20110718131210

Transaction Integrity Integrity of transactions: DB is updated by

processing transactions that results in changes to one or more DB records

When processing transactions, DBMS must ensure that the transactions follow four well-accepted properties – ACID Atomic Consistent Isolated Durable


Transaction Integrity Atomic

Transaction cannot be subdivided Once transaction is processed – changes are committed Transaction fails - aborted

Consistent Constraints that are true before the transaction must be

true after Isolated

Changes to DB not revealed to users until transaction is committed

Durable Changes are permanent – once committed no failure can

reverse the effect of the transaction

Transaction Integrity To maintain transaction integrity –

DBMS must provide facilities for the user or application program to define transaction boundaries – logical beginning and end of transaction.

BEGIN TRANSACTION..

UPDATEINSERT

.

.COMMIT


Backward Recovery (Rollback) DBMS backs out of or undo unwanted changes to

the DB – before images captured Reverse the changes made by transactions that

have aborted or terminated abnormally


Backward Recovery (Rollback)

Example: transfer 100 from CUSOMER A account to CUSTOMER B account Program reads the record for customer A and subtracts

100 from the account balance Program reads the record for customer B and adds 100

to the account balance. Program writes the updated record for A to the dbase. In attempting to write the record for B, program

encounters an error condition and cannot write the record.

An UNDO command – recovery manager to apply the before image for record A to restore acc balance to its original value.

Forward Recovery (Roll Forward) A technique that starts with an earlier copy of the

database. After images are applied to the database and the database is quickly moved forward to a later state.

The Rollforward is redoing the changes made by a transaction that is after the committed transaction and to over-write the changed value once again to ensure the consistency.

The time consuming logic of reprocessing each transaction does not have to be repeated

Only the most recent after-images need to be applied. DB record may have series of after image – most recent (good) after image is required for rollback


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BASIC RECOVERY TECHNIQUESROLLFORWARD

Difference in summary: Rollback :- Undoing the changes made

by a transaction before it commits or to cancel any changes to a database made during the current transaction

RollForward :- Re-doing the changes made by a transaction after it commits or to overwrite the changed value again to ensure consistency

Database Failure Responses Aborted transactions

Preferred recovery: rollbackAlternative: Rollforward to state just prior

to abort Incorrect data

Preferred recovery: rollbackAlternative 1: rerun transactions not

including inaccurate data updatesAlternative 2: compensating transactions –

human intervention

Database Failure Responses System failure (database intact)

Preferred recovery: switch to duplicate database

Alternative 1: rollbackAlternative 2: restart from checkpoint

Database destructionPreferred recovery: switch to duplicate

databaseAlternative 1: rollforwardAlternative 2: reprocess transactions

Disaster Recovery

Disaster Recovery Contingency plans to cater for disasters –

destroy/damage data center Natural disasters Planning for DR Develop a detailed DR plan Schedule regular test of plan Choose multi-disciplinary team to carry out

plan Fast backup data center – off site location Send back up copies to backup data center

Contingency Plan Contingency plan is established to deal

with unusual events that are not part of the normal daily routine

Contingency plans detail the response necessary to deal with the types of event that may occur

Contingency Plan A contingency plan should include :

who the key personnel are and how they can be contacted

if the key personnel are unavailable, a list of alternative personnel and how they can be contacted

who decides that a contingency exists and how that is decided

the technical requirements of transferring operations elsewhere

the operational requirements of transferring operations elsewhere

any outside contacts who may help whether any insurance exists to cover the situation

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Database BACKUP & recovery