Data Storage in SQL Server
Within the data files, SQL Server stores data in 8KB blocks of contiguous disk space, known as pages. The page is the minimum unit of IO when SQL Server transfers data to and from disk. Each of these data pages has the capacity to store up to 8060 bytes of data (8192–132 for overhead), and a single row cannot span more than one page. This is an important consideration when calculating disk usage. For example, if a table has an average row size of 4050 bytes, only one row would fit per page, essentially doubling the disk space required to store the data.
In an effort to reduce internal operations and increase IO efficiency, SQL Server, when allocating space to a table or index, allocates space in extents. An extent
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