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 the 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:

• Deletion is not cleansing

  When you delete a file from the system, the operating system simply marks it as overwritable, but does not immediately erase the underlying data. Block-level backup accurately records such unerased data, so the backup size does not decrease immediately after a file is deleted.

• Invisible management data

  In addition to file content, file systems also contain management information (metadata) such as permissions and directory structure. Although this data is not counted in the size of individual files, it occupies physical space and must be backed up to ensure complete data restoration.

• Fragmentation amplification due to scattered data (common for disk-level backups in the industry)

  Block-level backup operates on fixed-size data blocks. If file data is stored in a fragmented manner on the disk, more non-empty data blocks need to be recorded during backup, resulting in amplified backup capacity.

Assume that a disk has a total capacity of 100 GB and is initially full of data:

• Scenario 1: First full backup after file deletion

  You delete a 10 GB file. The available disk space increases, but the underlying data remains there. When a full backup is performed, the backup capacity is still 100 GB (because the underlying data has not been erased).

• Scenario 2: Subsequent incremental backup

  You then add 10 GB of new data and modify 5 GB of existing data.

  An incremental backup records all changes (new data, modified data, and underlying deleted data). The data volume to be backed up is at least 15 GB. If the new and modified data is fragmented on the disk, the data volume will be larger.

Result:

The total space occupied by the two backups is at least 115 GB (100 GB + 15 GB), which is much greater than the actual 90 GB of data on the disk.