SQL Data page and Extent.


Data Page:  The size of the data page is 8KB. Data rows are put on the page serially, starting immediately after the header. A row offset table starts at the end of the page. And each row offset table contains one entry for each row on the page. Each entry records how far the first byte of the row is from the start of the page. The entries in the row offset table are in reverse sequence from the sequence of the rows on the page.

Extent:  Extents are the basic unit in which space is managed. An extent is eight contiguous pages, or 64 KB. This means SQL Server database have 16 extents per megabyte (1MB). To make its space allocation efficient SQL Server does not allocate whole extents to table with small amounts of data. SQL Server has two types of events.
  1 Page->8KB
  8 Pages->8*8=64KB(One Extent)
    16*64=1024KB(1MB)
 

 

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