Download Maintenance

Week 7 : Chapter 7 Agenda
Maintenance Plan:
• Why do maintenance?
• Overview
• Maintenance Plan wizard
• DBCC maintenance commands
Why do Maintenance?
• Routinely want to make a backup copy of
database in case of hardware problems or
data corruption
• Data in a database becomes fragmented
over time as data is added, modified and
deleted
• Should verify integrity of data occasionally
• Normally want to perform these tasks when
you choose – not as the result of a crisis (just
like car maintenance; don’t want to be stuck
on the side of a highway with no oil in
engine)!
Maintenance Overview
• Day-to-day operation
• Very important responsibility of a DBA
• If you do maintenance nobody notices but if
you don’t do it everyone knows
• 2 types of maintenance:
- to keep database running  most important
- to improve performance
• Database maintenance can be fully
automated – there is a wizard for almost
every task
Database Maintenance Wizard
 Primary tool for database maintenance
 Graphical tool
 Can create a scheduled maintenance
plan for each database that performs:
• Backups (most important!)
• Optimization
• Integrity Check
• Reporting
Maintenance Planning
• Normally use wizard to create a
maintenance plan for each database
• Best to have one plan per database but can
choose more than 1 database per plan
• Plans are scheduled separately as required
by application
• Very important to create a separate
maintenance plan for critical system
databases: master, msdb
Maintenance Plan
• A Maintenance Plan consists of a group of
jobs and schedules called a plan
• A plan can be edited after you create it
• Very simple tool to use - no excuse not to
use it
• Within a job you can perform:
• Backups
• Optimization
• Integrity Check
• Reporting
Database Backups
 Most important part of maintenance plan:
permits recovery of data from backup copy
 Backs up database files or logs to a tape or
hard drive
 Hard drive location can be broken into several
directories for each database
 Can automatically delete older backups after
a certain time
 Wizard records all activities and sends
notification
 Report can be sent to disk, a central server, a
history table or to an operator
Optimization
• Improves database performance
• Database gets fragmented when data is
inserted and deleted (same as files on a
hard drive, see last slides for illustration)
• Optimization causes the following to be
performed that can improve database
performance:
• Examine and minimize fragmentation of
data
• Update database statistics
• Remove excess free space from
database
Data Fragmentation
• Database get fragmented when data is
inserted or deleted
• You can select how much free space for new
data will be left when defragmenting is done
• If database is primarily for data entry (OLTP)
then it is best to leave significant free space at
the end of each page so that related data
stays together
• If database is composed of data that is read
more often than being written (OLAP) such as
a report server then leave lower percentage of
page space free.
Update database statistics
 Statistics are samples of data used to assist
SQL Server search engine in locating data
 As data is added and deleted these statistics
become outdated
 Can keep statistics updated 2 ways:
• Use wizard to schedule update of statistics
periodically (this may slow performance if
statistics are out of date because data
would not be retrieved efficiently)
• Set database option that automatically
updates statistics (this slows performance
because statistics are continually updated)
Remove excess free space
• This setting in the wizard permits shrinking
the database at a specific recurring time.
• Automatic shrink of database can be set
as an option
Database Integrity Check
• SQL Server is a very stable RDBMS
• However should check integrity of a
database periodically
• Integrity checks validity of defined database
constraints (NN, PK, FK, CK, UN)
• Integrity check can correct minor errors
automatically
• May have to restore data from backup if
major integrity error is found (very rare!)
T-SQL Maintenance Commands
• T-SQL maintenance commands are the
Database Consistency Check commands
(DBCC)
• Command line utility
• Maintenance wizard actually uses these
commands
• Perform maintenance on Database Index and
file groups
• Commands include:
• DBCC CHECKDB
• DBCC CHECKTABLE
• DBCC DBREINDEX
DBCC CHECKDB Command
• CHECKDB command examines an entire
database for corruption (checks all tables
and indexes in database)
• Command can be run in diagnostic mode
• To correct any problem it has to be run in
single user mode
• Can use REPAIR_FAST or
REPAIR_ALLOW_DATA_LOSS switch
CHECKDB Switches
REPAIR_FAST Switch:
• Least amount of damage to database
• Quickly fixes any inconsistency
• Don’t lose any data
REPAIR_ALLOW_DATA_LOSS Switch:
• Most harmful switch but could save overall
database
• You can use this command on a database
that you plan to recover from tape backup
DBCC CHECKTABLE Command
• Checks for database corruption against a
table.
• You can use this command while people
are using the table except while
performing repair
DBCC DBREINDEX Command
 Command updates the indexes on a server
 Indexes are used for faster data access
 Indexes are stored like data pages and
become fragmented over time
 DBCC DBREINDEX command defragments
indexes
 Time required depends on:
• Number of indexes
• Size of indexes
• Load on server when running command
Physical structure of database
Insert Order 1 orderlines on Page 1:
Order 1
Item 1
Quantity
Order 1
Item 2
Quantity
Physical structure of database (ctd)
Insert Order 2’s orderlines on Page 1(blue) and
page 2 (white)
Order 1
Order 1
Order 2
Order 2
Order 2
Order 2
Order 2
Order 2
Order 2
Item 1
Item 2
Item 1
Item 2
Item 3
Item 4
Item 5
Item 6
Item 7
Client 1
Client 1
Client 2
Client 2
Client 2
Client 2
Client 2
Client 2
Client 2
Physical structure of database (ctd)
Add order 1 orderlines – no room on page 1 so place
on page 2 – data for order 1 is fragmented
Order 1
Item 1
Quantity
Order 1
Item 2
Quantity
Order 2
Item 1
Quantity
Order 2
Item 2
Quantity
Order 2
Item 3
Quantity
Order 2
Item 4
Quantity
Order 2
Item 5
Quantity
Order 2
Item 6
Quantity
Order 2
Item 7
Quantity
Order 1
Item 3
Quantity
Order 1
Item 4
Quantity
Physical structure of database (ctd)
Order 2 cancelled- delete Order 2’s orderlines –
leaves Order 1’s orderlines fragmented
Order 1
Item 1
Quantity
Order 1
Item 2
Quantity
Order 1
Item 3
Quantity
Order 1
Item 4
Quantity