理解overhead&reclaim&layout in computer

• Overhead

  Overhead is any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to perform a specific task.

  It is a special case of engineering overhead.

• Reclaim Wasted Space

  Reclaim means retrieve or recover.

  Azure SQL Database does not automatically shrink data files to reclaim unused allocated space due to the potential impact to database performance.

• Shrink a Database

  Shrinking data files recovers space by moving pages of data from the end of the file to unoccupired 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.

• Fragmented

  As data is modified in a database, the database and its indexes become fragmented.

  As indexes become fragmented, ordered data retrieval becomes less efficient and reduces database performance.

• Fragmentation

  Fragmentation is the task of dividing a table into a set of smaller tables.

  The subsets of the table are called fragments.

• Layout

  In computing, layout is the process of calculating the position of objects in space subject to various constraints.

• References

1. MicroSoft Azure Docs
2. I have deleted a lot of data from the database, but the disk space has not changed
3. MicroSoft SQL Docs
4. Database Index Defragmentation for SQL Server
5. Distributed DBMS - Design Strategies