Why Is My Backup Size Larger Than My Disk Size?

Symptoms

• There were files on a server, and the server was backed up. After deleting some files, the server was backed up again. By viewing the used vault capacity, we came to a conclusion that the backup size was not changed or even became bigger.

  For example, a user backed up a server containing 100 files, resulting in a used vault capacity of A. After deleting 10 files and running another backup, the used vault capacity became B. B may be the same as A or even larger.

• The ECS backup size is larger than the used disk space obtained from the file system.

Possible Causes

Possible causes are as follows:

• File system deletion: When a file is deleted from the operating system, the disk blocks it occupied are only marked as free space. The underlying data is not erased, so block-level backups still include this data.
• File system metadata overhead: File system management information (such as permissions and directory structures) occupies space. Although such information is not counted in the file size, it must still be backed up.
• Disk storage allocation: Disks are allocated in clusters, which can cause internal fragments when storing small files. In addition, hidden files (like hibernation files) also occupy physical space. As a result, the backup size may be larger than the size reported by the file system.
• Block-level backup: The backup size is calculated by summing all non-empty blocks, based on the disk's fixed block size. If file data is scattered across the disk, more blocks are marked non-empty, increasing backup size. (Disk-level backups in the industry are typically larger than the size reported by the file system.)
  

  Suppose a disk has a capacity of 100 GB and contains 100 GB of data. If 10 GB of data is deleted, the underlying data is not erased. The first backup is therefore a full backup by default and still occupies 100 GB. Later, if 10 GB of new data is added and 5 GB of existing data is modified, the second incremental backup must include all new, modified, and deleted data, resulting in at least 15 GB of backup. Therefore, the total space occupied by the two backups is at least 115 GB.