Differential Changed Map Pages
The seventh page (page number 6), and every 511,232nd page thereafter, in the database file is the differential changed map (DCM) page. This page keeps track of which extents in a file have been modified since the last full database backup. When an extent has been modified, its corresponding bit in the DCM is turned on. This information is used when a differential backup is performed on the database. A differential backup copies only the extents changed since the last full backup was made. Using the DCM, SQL Server can quickly tell which extents need to be backed up by examining the bits on the DCM pages for each data file in the database. When a full backup is performed for the database, all the bits are set back ...
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