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 What is an Allocation Structure ?
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HamidReza66
Starting Member

3 Posts

Posted - 07/13/2014 :  21:04:04  Show Profile  Reply with Quote
I Was Studied About Dbcc Command Family in Sql Server , During Study About Dbcc CheckAlloc , I Saw This Sentence " Checks the consistency of disk space allocation structures " , about 3 week i am searching About meaning of an allocation structure without any resaults , please help me ...
What is an Allocation Structure ?

robvolk
Most Valuable Yak

USA
15688 Posts

Posted - 07/14/2014 :  06:17:59  Show Profile  Visit robvolk's Homepage  Reply with Quote
You can learn about pages and extents here:

http://technet.microsoft.com/en-us/library/ms190969%28v=sql.105%29.aspx

You can also learn a lot more about SQL Server internals at SQL Skills, particularly Paul Randal's blog:

http://www.sqlskills.com/blogs/paul/
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HamidReza66
Starting Member

3 Posts

Posted - 07/14/2014 :  08:08:07  Show Profile  Reply with Quote
Thank you my friend , I want to know what is the concept of Allocation Structure,
page & Extent are allocation structures for DataBase , But Do You Know which structures are "Disk space allocation structures" ?
Thank You ,
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robvolk
Most Valuable Yak

USA
15688 Posts

Posted - 07/14/2014 :  23:37:06  Show Profile  Visit robvolk's Homepage  Reply with Quote
Pages and extents are the only allocation structures in SQL Server. The transaction log has a different structure, you'll find more details on SQLSkills. I wouldn't call it an allocation structure since it's not used for reading data under normal operations.
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Shanky
Yak Posting Veteran

United Kingdom
84 Posts

Posted - 07/17/2014 :  06:38:33  Show Profile  Reply with Quote
Allocation structures are basically the pages which stores information about SQL server data and index pages. They are
1.PFS pages: Stores information about amount of free space on each page and allocation details of the page whether it is allocated or not.It also tracks whether page has ghost records or not.
2.GAM pages: Global Allocation Map pages which stores information about uniform extents whether they are available for allocation or not
3 SGAM pages: Shared Global Allocation Map this stores information about whether a mixed extent is available for being allocated or not.

feel free to ask if you have any more doubts

Regards
Shanky
http://social.technet.microsoft.com/wiki/contents/articles/24253.list-of-articles-by-shanky.aspx

Edited by - Shanky on 07/17/2014 06:39:24
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