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Data storage

Data is stored in what are called as Pages, pages are of 8 Kilobytes.
Pages are then grouped in eight, which is called extent. 
Significance of Extent : Whenever data is read, it is read in 64K size.
That means it grabs a complete extent.
Refer to below diagram.
Above image shows a single row being stored in a page. (Whole of the 8 K
 is not available for data storage, some of it is used for house-keeping purpose)

Fill Factor

This parameter decides how much free space we have to keep in a data page, when
index are rebuilt.