Shrink Operation- Internally what happens?

Shrinking data files recovers space by moving pages of data from the end of the file to unoccupied space closer to the front of the file. When enough free space is created at the end of the file, data pages at end of the file can be deallocated and returned to the file system.

Limitations and Restrictions:

• The database cannot be made smaller than the minimum size of the database. The minimum size is the size specified when the database was originally created, or the last explicit size set by using a file-size-changing operation, such as DBCC SHRINKFILE. 
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• For example, if a database was originally created with a size of 10 MB and grew to 100 MB, the smallest size the database could be reduced to is 10 MB, even if all the data in the database has been deleted.
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• You cannot shrink a database while the database is being backed up. Conversely, you cannot backup a database while a shrink operation on the database is in process.

• DBCC SHRINKDATABASE will fail when it encounters an xVelocity memory optimized columnstore index. Work completed before encountering the columnstore index will succeed so the database might be smaller. 
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• To complete DBCC SHRINKDATABASE, disable all columnstore indexes before executing
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• DBCC SHRINKDATABASE, and then rebuild the columnstore indexes.

Recommendations:

• To view the current amount of free (unallocated) space in the database.
• Consider the following information when you plan to shrink a database: 
• A shrink operation is most effective after an operation that creates lots of unused space, such as a truncate table or a drop table operation.
• Most databases require some free space to be available for regular day-to-day operations. If you shrink a database repeatedly and notice that the database size grows again, this indicates that the space that was shrunk is required for regular operations. In these cases, repeatedly shrinking the database is a wasted operation.
• A shrink operation does not preserve the fragmentation state of indexes in the database, and generally increases fragmentation to a degree. This is another reason not to repeatedly shrink the database.
• Unless you have a specific requirement, do not set the AUTO_SHRINK database option to ON.