sql server physical architecture

8/8/2019 SQL Server Physical Architecture

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Physical database architecture

Training Division

New Delhi


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? The fundamental unit of data

storage in Microsoft SQLServer is the page.

? In SQL Server version 7.0 the size

of pages is 8 KB which meansthere can be 128 pages per

megabyte.

? The start of each page is a 96

byte header used to store

information

? such as the type of page,

? the amount of free spaceon the page,

? the object ID of the

object owning the page.

? The body of the page is of 8096

bytes.

Page Header

Body8K

page

A

B

A- 96 byte header

B- 8096 byte body

Pages


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?Data Data rows with all data

except text,ntext, andimage data

? Index Index entries

?Text/Image text, ntext, and image data

?Global Allocation maps Information aboutallocated extents

?Page Free Space Information about free

space available on pages

? Index Allocation map Information about extents

used by a table or index

Types of Pages


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Data Pages

? Data pages contain all the data in

data rowsexcept text,ntext, andimage data, which are stored in

separate pages.

? Data rows are placed serially on the

page starting immediately after the

header.

? Rows cannot span pages in SQLServer.

? In SQL Server 7.0,the maximum

amount of data contained in a singlerow is 8060 bytes, not including text,

ntext, and image data.


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Row offset table

? Starts at the end of the page and determines the location of arow within a page.

? Contains on entry for each row on the page and each entryrecords how far the first byte of the row is from the start ofthe page.

? The entries in the row offset table are in reverse sequencefrom the sequence of the rows on the page.


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Deleting Name1

Inserting Name4 Inserting Name5


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Index Pages

Stores the index pages .

An index page has the same layout as the data page.

Row in an index page consists of the index key and the

pointer to the page at the next lower level.


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Extents

? Extents are the basic unit in which space isallocated to tables and indexes.

? An extent is 8 contiguous pages, or 64KB(databases

have 16 extents per MB. )


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SQL Server 7.0 has two types of extents:

?Uniform extentsare owned by a single object; alleight pages in the extent can only be used by the

owning object.

?Mixed extentsare shared by up to eight objects.

Page address

Page address


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Log Data

Stored in a physically separate location from the data.

No longer stored in the system table.

Therefore ,does not compete for memory resources.

Physically stored as one or several log files which SQL

Server stores as series of records.


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Text and Image data

Text , ntext and image datatypes used.

Each column for a row of these types store upto 2 GB.

In the data page , there is a 16 byte pointer which

points to the location of the text or image data.

A table has one collection of pages to hold the text and

image data. (Stored in sysindexes for table ,indid=255)


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Page Free Space Pages

Page Free Space (PFS) pages record

? Whether an individual page has been allocated,

? Amount of space free on each page.

? Each PFS page covers 8,000 pages.

For each page, the PFS has a bitmap recording whether the

page is

? empty

? 1-50% full,

? 51-80% full,

? 81-95% full,

? or 96-100% full.


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Global Allocation Map pages

GAM pages record

? What extents have been allocated.

? Whether they have been allocated toobjects & indexes

? Whether the allocation has been for

uniform or mixed extents


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There are two types of Global Allocation Maps:

Global Allocation Map(GAM)

? Keeps track of allocated extents

irrespective of whether the

allocation is for mixed or uniform

extent.

? The GAM has one bit for eachextent in the

? If the bit is 1, the extent is free;

? if the bit is 0, the extent is

allocated.

Shared Global Allocation Map

(SGAM)? Records what extents are currently

used as mixed extents and have at

least one unused page.

? The SGAM has one bit for each extent

in the interval it covers.

? If the bit is 1, the extent is being

used as a mixed extent and has

free pages;

?

If the bit is 0, the extent is notbeing used as a mixed extent,or it

is a mixed extent whose pages

are all in use.

Both GAM & SGAM covers 64,000 extents, or nearly 4 GB of data


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Extent Usage GAM Bit SGAM Bit

Free 1 0

Uniform 0 0

Mixed ,with no free pages 0 0

Mixed ,with free pages 0 1


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? A new table or index is

allocated pages from mixedextents.

? When the table or index grows tothe point that it has eight pages, it

is switched to uniform extents.

SQL Server 7.0 does not allocate entire extents to tables with smallamounts of data inorder to make its space allocation efficient,


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Database Files and Filegroups

?A database is mapped over a set of operating systemfiles.

?These files are created at the same time as the

database is created.

?Minimum of two operating system files are created for

each database created.

? Primary data file

? Log file


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SQL Server 7 allows the following three types of

database files:

Primary data files :

Every database has one primary data file that keeps track of all the

rest of the files in the database,in addition to storing data.By

convention,the name of a primary data file has the extension MDF.

Secondary data files:

A database might have zero or more secondary data files.By

convention, the name of a secondary data file has the extension

NDF.

Log files :

Every database will have at least one lof file that contains the

information necessary to recover all the transactions in a

database.By convention, a log file will have the suffix LDF.


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Maximum size for a

database file

Maximum size of a

log file

32 TB

4 TB


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SQL Server 7.0 databases have three types of files:

? Primary data files

?Is the starting point of the database ,

?Points to the rest of the files in the database,

?Every database has one primary data file.

?Recommended file extension for primary data filesis .mdf.


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?Secondary data files

? Comprise all of the data file other than the primarydata file.

? Some databases may not have any secondarydata files, while others have multiple secondary

data files.

? The recommended file extension for secondarydata files is .ndf.


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?Log files

? Hold all of the log information used to recover thedatabase.

? There must be at least one log file for each database,although there can be more than one.

? The recommended file extension for log files is .ldf.


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On creation of a database say for eg : Training , the

two files that are created are :

C:\MSSQL7\data\training_Data.MDF A

C:\MSSQL7\data\training_Log.LDF B

where A is the primary data file ,

and B is the log file.

The information of the database files is contained in the tablecalled sysfiles .


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Sysfiles table

File id

groupid

size

maxsize

growth

perf

name

filename

Database identification number which is unique for each database

Identification of the filegroup to which the file belongs

Size of the file in pages

Max size of file.0-no autogrowth,1-autogrowth till disk file

Autogrowth increment in pages or percentage of file size

Reserved for future use

Logical name of file

The physical name of the file, including path


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?SQL Server 7.0 files can grow automatically from their originally specified

size.

?When you define a file, you can specify a growth increment.

?Each time the file fills, it increases its size by the growth increment.

? If there are multiple files in a filegroup, they do not autogrow until all the filesare full.

?Each file can also have a maximum size specified.

? If a maximum size is not specified, the file can continue to grow until it has

used all available space on the disk.

?The user can let the files autogrow as needed to lessen the administrative

burden of monitoring the amount of free space in the database and allocating

additional space manually.


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Points to remember :

If the database must never be allowed to grow beyond its

initial size,then set the maximum growth size of the databaseto zero.

This will prevent the database files from growing. If thedatabase files fill with data, no more data is added until moredata files are added to the database or existing files are

expanded.Fragmentation of Files

Allowing files to grow automatically can cause fragmentationof those files if a large number of files share the same disk.

Therefore, it is recommended that files or filegroups becreated on as many different available local physical disks aspossible.

Place objects that compete heavily for space in differentfilegroups.


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Disk Management Techniques

SQL Server can

? Allow the database file to grow

automatically

? Shrink the size of the database if the spaceis not needed


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Creating a database specifying the primary,secondary and log files with

autogrowth feature.

create database training

on (name=training_data1,

filename=c:\sql_data\training1.mdf,

size=50,

maxsize=100,

filegrowth=10),(name=training_data2,

filename=d:\sql_data\training2.ndf,

size=100,

filegrowth=20),

log on (name=training_log,

filename=e:\sql_data\training_log.ldf,

size=50,

filegrowth=20%)

go


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? Each file within a database can be shrunk to remove unused pages.

? Both data and transaction log files can be shrunk.

? The database files can be shrunk manually, either as a group or

individually and can also be set to shrink automatically at given

intervals.

? Shrinking activity occurs in the background and does not affect any

user activity within the database.

Shrinks the size of the data files in the specified database.

DBCC SHRINKDATABASE( database_name [, target_percent]

[, {NOTRUNCATE | TRUNCATEONLY}]

)

Shrinking of databases


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Shrinking size of database file

Shrinks the size of the specified data file or log file for therelated database.

DBCC SHRINKFILE

{file_name | file_id }{ [, target_size]

| [, {EMPTYFILE | NOTRUNCATE | TRUNCATEONLY}]

})


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Database filegroups

A database comprises of :? A primary filegroup and

? Any user-defined filegroups.

? Default filegroups.

The primary filegroup contains the :

? Primary data file and

?

Any other files that are not put intoanother file group.

? All pages for the system tables areallocated in the primary file group.


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User defined file group

These are filegroups that are specified using theFILEGROUP keyword in a CREATE DATABASE or ALTER

DATABASE statement, or on the property page within SQLServer Enterprise Manager.

Default filegroup

They contains the pages for all tables and indexes that do

not have a filegroup specified when they are created. Ineach database, only one filegroup at a time can be thedefault filegroup. If no default filegroup was specified, itdefaults to the primary filegroup.


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Some important facts about file groups:

? No file can be a member of more than one

filegroup.? Log files are never a part of a filegroup.

? Files in a filegroup will not autogrow unless thereis no space available on any of the files in the

filegroup.

? A maximum of 256 file groups can be created per

database, and file groups can contain only datafiles;

? It is not possible to move files to a differentfilegroup once the files have been added to thedatabase.


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Advantages of filegroups:

File groups allow files to be grouped together for administrative

and data allocation/placement purposes.

For example, three files (data1.ndf, data2.ndf, and data3.ndf) can becreated on three disk drives, respectively, and assigned to thefilegroup fgroup1.

A table can then be created specifically on the filegroup fgroup1.

Queries for data from the table will be spread across the three disks,thereby improving performance.

The same performance improvement can be accomplished with asingle file created on a RAID (redundant array of independent disks)stripe set.

Files and filegroups, however, allow you to easily add new files onnew disks.

Additionally, if your database exceeds the maximum size for asingle Microsoft Windows file, you can use secondary data files toallow your database to continue to grow.


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By creating a filegroup on a specific disk or

RAID (redundant array of independent disks)device, you can control where tables andindexes in your database are physically located.

Reasons for placing tables and indexes on specificdisks include:

?Improved query performance.

?Parallel queries.


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The following example creates a database with a primarydata file, a user-defined filegroup, and a log file. Theprimary data file is in the primary filegroup and the user-

defined filegroup has two secondary data files. AnALTER DATABASE statement makes the user-definedfilegroup the default. A table is then created specifyingthe user-defined filegroup.


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CREATE DATABASE training

ON PRIMARY

( NAME=Trg_Primary',

FILENAME='c:\mssql7\data\Trg_Prm.mdf',

SIZE=4,

MAXSIZE=10,

FILEGROWTH=1),

FILEGROUP Trg_FG1

( NAME = Trg_FG1_Dat1',

FILENAME = 'c:\mssql7\data\Trg_FG1_1.ndf',

SIZE = 1MB,

MAXSIZE=10,

FILEGROWTH=1),( NAME = Trg_FG1_Dat2',

FILENAME = 'c:\mssql7\data\Trg_FG1_2.ndf',

SIZE = 1MB,

MAXSIZE=10,

FILEGROWTH=1)

LOG ON

( NAME=trg_log',FILENAME='c:\mssql7\data\Trg.ldf',

SIZE=1,

MAXSIZE=10,

FILEGROWTH=1)

GO


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An ALTER DATABASE statement makes the user-

defined filegroup the default.

ALTER DATABASE Trg

MODIFY FILEGROUP Trg_FG1 DEFAULT

GO

A table is then created specifying the user-

defined filegroup.

USE Trg

CREATE TABLE TrgTable

(par_id int PRIMARY KEY,

par_nm char(8) )

ON Trg_FG1

GO


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Indexes

Indexes can be of two types:

1. Clustered Index :

?Data rows are sorted and stored in the table based on their key values.

?There can only be one clustered index per table because the data rows

themselves can only be sorted in one order.

?

Clustered indexes are efficient for finding rows.?

?The data rows form the lowest level of the clustered index.


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2. Non Clustered Index :

Nonclustered indexes have a structure that is completely separate from

the data rows.

The lowest rows contain the nonclustered index key values and each key

value entry has pointers to the data rows containing the key value.

The data rows are not stored in order based on the nonclustered key.


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Two types of tables :

1.Clustered tables

Are tables that have a clustered index.

The data rows are stored in order based on the clusteredindex key. The data pages are linked in a doubly-linkedlist. The index is implemented as a B-tree index structurethat supports fast retrieval of the rows based on theirclustered index key values.

2. Heaps

Are tables that have no clustered index.The data rows are not stored in any particular order, andthere is no particular order to the sequence of the datapages. The data pages are not linked in a linked list.


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Maximum Capacity SpecificationsThis table specifies the maximum sizes and numbers of various objectsdefined in Microsoft SQL Server databases or referenced in Transact-SQL

statements.

Object SQL Server 7.0

Batch size 65,536* Network Packet SizeBytes per short string column 8000

Bytes per text, ntext, or image column 2 GB-2

Bytes per GROUP BY, ORDER BY 8060Bytes per index 900

Bytes per foreign key 900

Bytes per primary key 900Bytes per row 8060

Bytes in source text of a stored procedure Lesser of batch size or 250 MB

Clustered indexes per table 1

Columns in GROUP BY, ORDER BY Limited only by number of bytesColumns or expressions in a GROUP BY

WITH CUBE or WITH ROLLUP statement 10Columns per index 16

Columns per foreign key 16

Columns per primary key 16Columns per base table 1024


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Columns per SELECT statement 4096Columns per INSERT statement 1024Connections per client Max. value of configured

connections

Database size 1,048,516 TB

Databases per server 32,767Filegroups per database 256

Files per database 32,767

File size (data) 32 TBFile size (log) 4 TB

Foreign key table references per table 253Identifier length (in characters) 128

Locks per connection Max. locks per server

Locks per server 2,147,483,647 (static)40% of SQL Server

memory (dynamic)Nested stored procedure levels 32

Nested subqueries 32

Nested trigger levels 32


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Nonclustered indexes per table 249

Objects concurrently open in a server* 2,147,483,647Objects in a database* 2,147,483,647

Parameters per stored procedure 1024

REFERENCES per table 63Rows per table Limited by available

storage

SQL string length (batch size) 128* TDS packet size

Tables per database Limited by number of

objects in a database

Tables per SELECT statement 256

Triggers per table Limited by number ofobjects in a database

UNIQUE indexes or constraints per table 249 nonclustered and 1

clustered

* Database objects include all tables, views, stored procedures, extended storedprocedures, triggers, rules, defaults, and constraints. The sum of the number of

all these objects in a database cannot exceed 2,147,483,647.

sql server physical architecture

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