Backup and Recovery

Question 1

What is Backup?

Answer 1

A Backup of a database is a representative copy of data containing all necessary
contents of a database such as data files and control files

Question 2

two major types of backup?

Answer 2

Physical Backup: A copy of physical database files such as data, control files,
log files, and archived redo logs.
▷ Logical Backup: A copy of logical data that is extracted from a database
consisting of tables, procedures, views, functions, etc

Question 3

What is recovery?

Answer 3

Recovery is the process of restoring the database to its latest known consistent state
after a system failure occurs.

Question 4

What is a Database Logs?

Answer 4

◦ A Database Log records all transactions in a sequence. Recovery using logs is quite
popular in databases
◦ A typical log file contains information about transactions to execute, transaction
states, and modified values

Question 5

What situations warrant Backup?

Answer 5

Disaster Recovery
Client Side Changes
Auditing
Downtime

Question 6

What are the Types of Backup Data?

Answer 6

1. Business Data
2. System Data (Environment/ configuration)
3. Media

Question 7

What are the types of Backup Strategies

Answer 7

1. Full Backup
2. Incremental Backup
3. Differential Backup

Question 8

What is Full Backup?

Answer 8

• Full Backup backs up everything. This is a complete copy, which stores all the objects
  of the database: tables, procedures, functions, views, indexes etc. Full backup can
  restore all components of the database system as it was at the time of crash.
• A full backup must be done at least once before any of the other type of backup

Question 9

What is Incremental Backup?

Answer 9

Incremental backup targets only those files or items that have changed since the last
backup. This often results in smaller backups and needs shorter duration to complete
the backup process.

Question 10

What is Differential Backup?

Answer 10

Differential backup backs up all the changes that have occurred since the most recent
full backup regardless of what backups have occurred in between

Question 11

What is Hot Backup?

Answer 11

Hot backup refers to keeping a database up and running while the backup is
performed concurrently

Question 12

What is transaction logging?

Answer 12

• Transactional Logging is used in circumstances where a possibly inconsistent backup is taken, but another file generated and backed up (after the database file has been
  fully backed up) can be used to restore consistency.

Question 13

What are hot backup maiinly used for?

Answer 13

In regular database systems, hot backup is mainly used for Transaction Log Backup.

Question 14

What are cold backup strategies and when are they preferred?

Answer 14

Cold backup strategies like Differential, Incremental are preferred for Data backup.

Question 15

What are the various failure classifications?

Answer 15

1. Transactional Failure (a. Logical Errors, b. System Errors)
2. System Crash (Fail-Stop Assumption)
3. Disk Failure

Question 16

What are recovery algorithms and what does it consist of?

Answer 16

Recovery algorithms have two parts
a. Actions taken during normal transaction processing to ensure enough info exists to recover from failures.
b. Actions taken after a failure to recover the database contents to a state that ensures atomicity, consistency and durability.

Question 17

Types of Storage Structure?

Answer 17

1. Volatile Storage
2. Non-volatile storage
3. Stable Storage(mythical, maintain multiple copies,basically a RAID system)

Question 18

What are some of the recovery mechanism?

Answer 18

1. Log based recovery system.
2. Shadow paging method.

Question 19

What is a log record?

Answer 19

The log is a sequence of log records, which maintains information about updateactivities on the database.
When transaction Ti starts, it registers itself by writing a record < Ti start > to the log
* Before Ti executes write(X), a log record < Ti
, X, V1, V2 > is written, where V1 is the
value of X before the write (old value), and V2 is the value to be written to X (new
value)
* When Ti finishes its last statement, the log record < Ti commit > is written

Question 20

What are the types of database modification schemes?

Answer 20

The immediate-modification scheme allows updates of an uncommitted transaction to be made to the buffer, or the disk itself, before the transaction commits.

The deferred-modification scheme performs updates to buffer/disk only at the time of transaction commit

Question 21

When are the undo and redo operations used?

Answer 21

The undo and redo operations are used in several different circumstances:
◦ The undo is used for transaction rollback during normal operation
▷ in case a transaction cannot complete its execution due to some logical error
◦ The undo and redo operations are used during recovery from failure

Question 22

How are log based transaction roll backs done normally?

Answer 22

Let Ti be the transaction to be rolled back
* Scan log backwards from the end, and for each log record of Ti of the form < Ti, Xj, V1, V2 >
◦ Perform the undo by writing V1 to Xj,
◦ Write a log record < Ti , Xj, V1 >
▷ such log records are called Compensation Log Records
* Once the record < Ti start> is found stop the scan and write the log record < Ti abort>

Question 23

What is repeating history?

Answer 23

such a redo redoes all the original actions including the steps that restored old value – Known as Repeating History

Question 24

What is checkpointing?

Answer 24

Streamline recovery procedure by periodically performing checkpointing
* All updates are stopped while doing checkpointing
a) Output all log records currently residing in main memory onto stable storage
b) Output all modified buffer blocks to the disk
c) Write a log record < checkpoint L > onto stable storage where L is a list of all transactions active at the time of checkpoint