Microsoft SQL Server 2008 R2 Unleashed- P116 pot

However, the Database Engine uses the file location information stored in the primary file to initialize the file location entries in the master database when attaching a database using

SQL Server uses the file location information visible in the sys.master_files catalog view

most of the time However, the Database Engine uses the file location information stored

in the primary file to initialize the file location entries in the master database when

attaching a database using the CREATE DATABASE statement with either the FOR ATTACH or

FOR ATTACH_REBUILD_LOG options

Every database can have three types of files:

Primary data file

Secondary data files

Log files

In addition, in SQL Server 2008, databases can also have FILESTREAM data files and

full-TABLE 34.1 The sysfiles Table

Column Name Description

file_id A file identification number that is unique within each database

file_guid GUID for the file

type File type (0=rows [that is, data files], 1=log, 2=FILESTREAM, 4=Full-text

catalogs prior to SQL Server 2008 type_desc Description of the file type (ROWS, LOG, FILESTREAM, FULLTEXT)

data_space_id 0 represents a log file; values > 0 represent the ID of the filegroup the

data file belongs to name The logical name of the file

filename The physical name of the file, including path

state File state (0 = OFFLINE, 1 = RESTORING, 2 = RECOVERING, 3 =

RECOVERY_PENDING, 4 = SUSPECT, 6 = OFFLINE, 7=DEFUNCT) state_desc Description of the file state (OFFLINE, RESTORING, RECOVERING,

RECOVERY_PENDING, SUSPECT, OFFLINE, DEFUNCT) size Current size of the file in 8KB pages

max_size Maximum file size in 8KB pages

growth File growth setting (0=fixed, >0=autogrow in units of 8KB pages or by

percentage if is_percent_growth is set to 1) is_media_read_only 1=file is on read-only media

is_read_only 1= file is marked read-only

is_sparse 1=file is a sparse file

is_percent_growth 1=growth of file value is percentage

Primary Data File

Every database has only one primary database file The location of the primary database

file is stored in the master database (visible via the filename column in the

sys.master_files view) When SQL Server opens a database, it looks for this file and then

reads from the file information on the other files defined for the database

The file extension for the primary database file defaults to mdf The primary database file

always belongs to the default filegroup It is often sufficient to have only one database file

for storing your tables and indexes (the primary database file) The file can, of course, be

created on a RAID partition to help spread I/O However, if you need finer control over

placement of your tables across disks or disk arrays, or if you want to be able to back up

only a portion of your database via filegroups, you can create additional, secondary data

files for a database

Secondary Data Files

A database can have any number of secondary files (in reality, the maximum number of

files per database is 32,767, but that should be sufficient for most implementations) You

can put a secondary file in the default filegroup or in another filegroup defined for the

database Secondary data files have the file extension.ndf by default

Following are some situations in which the use of secondary database files might be

beneficial:

You want to perform a partial backup A backup can be performed for the entire

database or a subset of the database The subset is specified as a set of files or

file-groups The partial backup feature is useful for large databases, where it is

impracti-cal to back up the entire database When recovering with partial backups, a

transaction log backup must also be available For more information about backups,

see Chapter 14, “Database Backup and Restore.”

You want more control over placement of database objects When you create a table

or index, you can specify the filegroup in which the object is created This could

help you spread I/O by placing your most active tables or indexes on separate

file-groups defined on separate disks or disk arrays

Creating multiple files on a single disk provides no real performance benefit but

could help in recovery If you have a 90GB database in a single file and have to

restore it, you need to have enough disk space available to create a new 90GB file If

you don’t have 90GB of space available on a single disk, you cannot restore the

data-base On the other hand, if the database was created with three files each 30GB in

size, you more likely will be able to find three 30GB chunks of space available on

your server

The Log File

Each database must have at least one log file The log file contains the transaction log

records of all changes made in a database (for more information on what is contained in

the transaction log, see Chapter 31, “Transaction Management and the Transaction Log”)

By default, log files have the file extension ldf

A database can have several log files, and each log file can have a maximum size of 32TB

A log file cannot be part of a filegroup No information other than transaction log records

can be written to a log file

For more information on the log file and log file management, see Chapter 31

File Management

In SQL Server 2008, you can specify that a database file should grow automatically as

space is needed SQL Server can also shrink the size of the database if the space is not

needed You can control whether to use this feature along with the increment by which

the file is to be expanded The increment can be specified as a fixed number of megabytes

or as a percentage of the current size of the file You can also set a limit on the maximum

size of the file or allow it to grow until no more space is available on the disk

Listing 34.1 provides an example of a database being created with a 10MB growth increment

for the first database file, 20MB for the second, and 20% growth increment for the log file

LISTING 34.1 Creating a Database with Autogrowth

CREATE DATABASE Customer

ON ( NAME=’Customer_Data’,

FILENAME=’D:\SQL_data\Customer_Data1.mdf’,

SIZE=50,

MAXSIZE=100,

FILEGROWTH=10),

( NAME=’Customer_Data2’,

FILENAME=’E:\SQL_data\Customer_Data2.ndf’,

SIZE=100,

FILEGROWTH=20)

LOG ON ( NAME=’Customer_Log’,

FILENAME=’F:\SQL_data\Customer_Log.ldf’,

SIZE=50,

FILEGROWTH=20%)

GO

The Customer_Data file has an initial size of 50MB, a maximum size of 100MB, and a file

increment of 10MB

The Customer_Data2 file has an initial size of 100MB, has a file growth increment of

The transaction log has an initial size of 50MB; the file increases by 20% with each file

growth The increment is based on the current file size, not the size originally specified.

When creating or expanding data files in SQL Server 2008, SQL Server uses fast file

initial-ization This allows for the fast execution of the file creation and growth With fast file

initialization, the space is added to the data file immediately, but without initializing the

logical pages in the data file with zeros The existing disk content in the data file is not

overwritten until new data is written to the files This provides a huge performance

advan-tage when a data file autogrows while an application is attempting to write data to the

database The application does not need to wait until the space is initialized; it can begin

writing to the database immediately

SQL Server also provides an option to autoshrink databases as well as manually shrink

databases However, shrinking a database is a resource-intensive process and should be

done only if it is absolutely imperative to reclaim disk space Also, if a data file is

constantly shrinking and growing, it can lead to excessive file fragmentation at the file

system level as well as excessive logical fragmentation within the file, both of which can

lead to poor I/O performance

Using Filegroups

All databases have a primary filegroup that contains the primary data file There can be

only one primary filegroup If you don’t create any other filegroups or change the default

filegroup to a filegroup other than the primary filegroup, all files will be in the primary

file group unless specifically placed in another filegroup

In addition to the primary filegroup, you can add one or more filegroups to the database,

and a filegroup can contain one or more files The main purpose of using filegroups is to

provide more control over the placement of files and data on your server When you

create a table or index, you can map it to a specific filegroup, thus controlling the

place-ment of data A typical SQL Server database installation generally uses a single RAID array

to spread I/O across disks and create all files in the primary filegroup; more advanced

installations or installations with very large databases spread across multiple array sets can

benefit from the finer level of control of file and data placement afforded by additional

filegroups

For example, for a simple database such as AdventureWorks, you can create just one

primary file that contains all data and objects and a log file that contains the transaction

log information For a larger and more complex database, such as a securities trading

system where large data volumes and strict performance criteria are the norm, you might

create the database with one primary file and four additional secondary files You can then

set up filegroups so you can place the data and objects within the database across all five

files If you have a table that itself needs to be spread across multiple disk arrays for

perfor-mance reasons, you can place multiple files in a filegroup, each of which resides on a

different disk, and create the table on that filegroup For example, you can create three

files (Data1.ndf, Data2.ndf, and Data3.ndf) on three disk arrays, respectively, and then

assign them to the filegroup called spread_group Your table can then be created

specifi-cally on the filegroup spread_group Queries for data from the table are spread across the

three disk arrays, thereby improving I/O performance

If a filegroup contains more than one file, when space is allocated to objects stored in that

filegroup, the data is stored proportionally across the files In other words, if you have one

file in a filegroup with twice as much free space as another, the first file has two extents

allocated from it for each extent allocated from the second file (extents and space

alloca-tion are discussed in more detail later in this chapter)

Listing 34.2 provides an example of using filegroups in a database to control the file

place-ment of the customer_info table

LISTING 34.2 Using a Filegroup to Control Placement for a Table

CREATE DATABASE Customer

ON ( NAME=’Customer_Data’,

FILENAME=’C:\SQLData\Customer_Data1.mdf’,

SIZE=50,

MAXSIZE=100,

FILEGROWTH=10)

LOG ON ( NAME=’Customer_Log’,

FILENAME=’C:\SQLData\Customer_Log.ldf’,

SIZE=50,

FILEGROWTH=20%)

GO

ALTER DATABASE Customer

ADD FILEGROUP Cust_table

GO

ALTER DATABASE Customer

ADD FILE

( NAME=’Customer_Data2’,

FILENAME=’G:\SQLData\Customer_Data2.ndf’,

SIZE=100,

FILEGROWTH=20)

TO FILEGROUP Cust_Table

GO

USE Customer

CREATE TABLE customer_info

(cust_no INT, cust_address NCHAR(200), info NVARCHAR(3000))

ON Cust_Table

GO

TABLE 34.2 The sys.filegroups System Catalog View

Column Name Description

name Name of the data space, unique within the database

data_space_id Data space ID number, unique within the database

type FG = Filegroup

type_desc Description of data space type: ROWS_FILEGROUP

is_default 1 = This is the default data space The default data space is used when a

file-group or partition scheme is not specified in a CREATE TABLE or CREATE INDEX statement

0 = This is not the default data space

filegroup_guid GUID for the filegroup.

NULL = PRIMARY filegroup

log_filegroup_id Not used; value is NULL

is_read_only

1 = Filegroup is read-only

0 = Filegroup is read/write

The CREATE DATABASE statement in Listing 34.2 creates a database with a primary database

file and log file The first ALTER DATABASE statement adds a filegroup A secondary

data-base file is added with the second ALTER DATABASE command This file is added to the

Cust_Table filegroup The CREATE TABLE statement creates a table; the ON Cust_Table

clause places the table in the Cust_Table filegroup (the Customer_Data2 file on the G: disk

partition)

The sys.filegroups system catalog view contains information about the database

file-groups defined within a database, as shown in Table 34.2

The following statement returns the filename, size in megabytes (not including autogrow),

and the name of the filegroup to which each file belongs:

SELECT

convert(varchar(30), sf.name) as filename,

size/128 as size_in_MB,

convert(varchar(30), sfg.name) as filegroupname

FROM sys.database_files sf

INNER JOIN sys.filegroups sfg

ON sf.data_space_id = sfg.data_space_id

go

filename size_in_MB filegroupname

- -

-Customer_Data 50 PRIMARY

Customer_Data2 100 Cust_table

FILESTREAM Filegroups

FILESTREAM storage is a new feature in SQL Server 2008 for storing unstructured data,

such as documents, images, and videos FILESTREAM storage helps to solve the issues with

using unstructured data by integrating the SQL Server Database Engine with the NTFS file

system for storing the unstructured data, such as documents and images, on the file system

with the database storing a pointer to the data Although the actual data resides outside

the database in the NTFS file system, you can still use Transact-SQL (T-SQL) statements to

insert, update, query, and back up FILESTREAM data, while maintaining transactional

consistency between the unstructured data and corresponding structured data with same

level of security

NOTE

To use FILESTREAM storage, you must first enable FILESTREAM storage at the

Windows level as well as at the SQL Server instance level You can enable FILESTREAM

at the Windows level during installation of SQL Server 2008 or at any time using SQL

Server Configuration Manager After you enable FILESTREAM at the Windows level, you

next need to enable FILESTREAM for the SQL Server instance You can do this either

through SQL Server Management Studio (SSMS) or via T-SQL

After you enabled FILESTREAM for the SQL Server instance, you can enable it for a

data-base by creating a FILESTREAM filegroup You can do this when the datadata-base is created (or

to an existing database) by adding a filegroup and including the CONTAINS FILESTREAM

clause Unlike regular filegroups, a FILESTREAM filegroup can contain only a single file

reference, which is actually a file system folder rather than an actual file The actual folder

must not exist (although the path up to the folder must exist); SQL Server creates the

filestream folder For example, in Listing 34.3, the code adds a FILESTREAM filegroup

called CustFSGroup and adds the folder G:\SQLData\custinfo_FS into the file group This

custinfo_FS folder is created by SQL Server in the G:\SQLData folder

LISTING 34.3 Using a Filegroup to Control Placement for a Table

ALTER DATABASE Customer

ADD FILEGROUP Cust_FSGroup CONTAINS FILESTREAM

ADD FILE

( NAME=custinfo_FS,

FILENAME = ‘G:\SQLData\custinfo_FS’)

to FILEGROUP Cust_FSGroup

GO

If you look in the G:\SQLData\custinfo_FS folder, you should see a Filestream.hdr file

and an $FSLOG folder The Filestream.hdr file is a FILESTREAM container header file that

should not be moved or modified

As you can see in the example in Listing 34.3, for FILESTREAM files or file groups, unlike

regular files, you do not specify size or growth information No space is preallocated The

file and filegroup grow as data is added to tables that have been created with

FILESTREAM columns

As you create tables with FILESTREAM columns, a subfolder is created in the filegroup

folder for each table The filenames are GUIDs Each FILESTREAM column created in the

table results in another subfolder created under the table subfolder The column subfolder

name is also a GUID At this point, there still are no actual files created That happens

after you start adding rows to the table A file is created in the column subfolder for each

row inserted into the table with a non-NULL value for the FILESTREAM column

For more information on creating and using tables with FILESTREAM columns, see

Chapter 42, “What’s New for Transact-SQL in SQL Server 2008.”

Database Pages

All information in SQL Server is stored at the page level The page is the smallest level of

I/O in SQL Server and is the fundamental storage unit Pages contain the data itself or

information about the physical layout of the data The page size is the same for all page

types: 8KB, or 8,192 bytes The pages are arranged in two basic types of storage structures:

linked data pages and index trees

Databases are divided into logical 8KB pages Within each file allocated to a database, the

pages are numbered contiguously from 0 to n The actual number of pages in the database

file depends on the size of the file Pages in a database are uniquely referenced by

specify-ing the database ID, the file ID for the file the page resides in, and the page number

within the file When you expand a database with ALTER DATABASE, the new space is

added at the end of the file, and the page numbers continue incrementing from the

previ-ous last page in the file If you add a completely new file, its first page number is 0 When

you shrink a database, pages are removed from the end of the file only, starting at the

highest page in the database and moving toward lower-numbered pages until the database

reaches the specified size or a used page that cannot be removed This ensures that page

numbers within a file are always contiguous

TABLE 34.3 Page Types

Page Type Stores

Data Data rows for all data except text, ntext, image, nvarchar(max),

varchar(max), varbinary(max), and xml data Row Overflow Data columns that cause a data row to exceed the 8,060 bytes per page

limit LOB Large object types (text, ntext, image, nvarchar(max), varchar(max),

varbinary(max), xml data, and varchar, nvarchar, varbinary, and sqlvariant when data row size exceeds 8KB)

Index Index entries and pointers

Global Allocation

Map

Information about allocated (used) extents

Page Free Space Information about page allocation and free space on pages

Index Allocation

Map

Information about extents used by a table or an index

Differential

Changed Map

Information about which extents have been modified since the last full database backup

Bulk Changed Map Information about which extents have been used in a minimally logged or

bulk-logged operation since the last BACKUP LOG statement

Body

Header

96 byte header

8096 bytes

8K PagE

(8192Bytes)

FIGURE 34.1 SQL Server page layout

Page Types

There are eight page types in SQL Server, as listed in Table 34.3

All pages, regardless of type, have a similar layout They all have a page header, which is

96 bytes, and a body, which consequently is 8,096 bytes The page layout is shown in

Figure 34.1

Data Pages

The actual data rows in tables are stored on data pages Figure 34.2 shows the basic

struc-ture of a data page

The following sections discuss and examine the contents of the data page

The Page Header

The page header contains control information for the page Some fields assist when SQL

Server checks for consistency among its storage structures, and some fields are used when

navigating among the pages that constitute a table Table 34.4 describes the more useful

fields contained in the page header

Header

Row Offset Table

.

96 118 140

…

…

Row 0

Row 1

Row 2

Byte Address Row ID

0 1 2

96

118

140

0

3 4 8095

FIGURE 34.2 The structure of a SQL Server data page