Legacy Backup Format Overview

The legacy backup format is a proprietary simple backup format for storing backup data of any type.

To learn more about the new backup format, refer to the New Backup Format chapter

The current backup format supports the following backup types:

  • File backup
  • Image-Based Backup
  • VMware Backup
  • Hyper-V Backup
  • Microsoft SQL Backup
  • Microsoft Exchange Backup

Support for Major Storage Providers

The legacy backup format is supported by major storage providers. The legacy backup supports the following features:

  • Compression
  • Encryption
  • Versioning
  • Block-Level backup type
  • Retention Policy


For backup data compression, a standard Gzip mechanism is used.


For encryption, Backup Agent uses the Advanced Encryption Standard specification. Different key lengths are supported as well.


The advanced mode supports backup data versioning. Each time a backup is made, backup content is compared with the previous one and in case of difference, a new version is uploaded. Object version management is done via Retention Policy.

Block-Level Backup Type

Block-level is a backup approach where instead of a full copy backup (full backup) to backup storage, a backup sequence of a first full backup and following incremental backups is established. A backup source is divided by blocks and only modified blocks are subject to upload to backup storage.

Modification date is a criteria for deciding if a file or folder is a subject of backup or not. If a file was modified after the last backup time, it is divided by blocks, then blocks are compared. Modified blocks are uploaded to backup storage.

Block size varies for different kinds of backup sources. For a file-level backup, the block size depends on the file size:

  • The block size equals 128 kB if the file size is less than 512 GB
  • The block size equals 256 kB if the file size is less than 1024 GB
  • The block size equals 512 kB if the file size exceeds 1024 GB

For image-based, Microsoft Exchange, and VMware backups, the block size is 1 MB.

You always can run a full backup as well to be able to purge outdated file versions according to the regular retention policy settings. The backup service cannot delete a previous file version unless a newer full backup is uploaded to the storage, because this version is used for restoring the file.

Block-level backup is enabled by default in Microsoft Exchange backup type

Retention Policy

Multiple version support allows to set up a flexible retention policy for backup data.

Thus, You can make a backup service to delete file versions that were modified or uploaded to your backup storage more than a certain number of days ago. Once this retention policy option is enabled, you can also keep the last version of each file regardless of when they were backed up. Also, the maximum number of versions is subject to be specified by the user. According to these settings, the specified number of file versions are kept on backup storage. Along with version settings, you can specify a period for which locally deleted files should be kept in the backup storage before purging.