To streamline the backup process, several backup types are available. In most cases, a backup cycle consists of periodic full backups (for example, weekly) with more frequent incremental backups (for example, daily). Differential and synthetic full backups provide additional flexibility.
Full backups of entire virtual machines provide the most comprehensive protection of data; but they also consume the most time and resources.
Backups and restores of entire virtual machines are supported for all guest operating systems supported by VMware.
An incremental backup contains only data that is new or has changed since the last backup, regardless of the type. On average, incremental backups consume far less media and place less of a burden on resources than full backups.
The following illustration shows the relationship between full and incremental backups. For simplicity, assume there is a file system that contains six files as represented in the figure.
Backup #1 is a full backup and writes all the data, changed and unchanged, to the backup media. Backups #2 through #n-1 are incrementals and only back up files that have changed since the time of the last backup, regardless of the type. For example, files A, B, and E changed after the full backup and were backed up in Backup #2. Files A and D changed after Backup #3 and were backed up in Backup #4. File F did not change; it was not backed up in any of the incremental backups, but it was included in both full backups.
During incremental backups, Changed Block Tracking is used to optimize backups of virtual machines by reading only the allocated and modified portions of a virtual disk. Changed Block Tracking is automatically enabled for virtual machines running on hardware version 7 or later.
A differential backup contains only data that is new or has changed since the last full backup. Like incremental backups, differential backups, on average, consume less media and place less of a burden on resources than full backups. Differential backups are cumulative. This means that each differential backup contains all changes accumulated since the last full backup. Each successive differential backup contains all the changes from the previous differential backup.
The following illustration shows how differential backups work. For simplicity, assume there is a file system that contains six files as represented in the figure.
Backup #1 is a full backup and writes all data to the backup media. Backups #2 through #n-1 are differential backups and only back up files that changed since the time of the last full backup. For example, files A, B, and E changed after the full backup and were backed up in Backup #2 as well as all subsequent differential backups. File C changed sometime after Backup #2 and was backed up in Backup #3 and all subsequent differential backups. File F did not change; it was not backed up in any of the differential backups, but it was included in both full backups.
A synthetic full backup is a synthesized backup, created from the most recent full backup and all subsequent incremental or differential backups. The resulting synthetic full backup is identical to a full backup for the subclient.
Unlike full, incremental, and differential backups, a synthetic full backup does not actually transfer data from a client computer to the backup media, and does not use any resources on the client computer.
Synthetic full backups are media-based; they read backup data from one media and simultaneously write (restore) the result to the new active media within the same media group. As a result, synthetic full backups require that at least two media drives for the same storage policy be available at the time the job is started. Synthetic full backups cannot be performed on subclients where the storage policy is associated with a stand-alone drive.
An incremental backup can be run either before or after a synthetic full backup.
- Running an incremental backup before the synthetic full ensures that any new or recently changed data is included in the synthetic full.
- Running an incremental backup after the synthetic full ensures that any new or recently changed data since the backup that occurred prior to the synthetic full, but was not included in the synthetic full, is backed up by the incremental.
When a storage policy copy is deduplicated, synthetic full backups can be created in an accelerated mode to significantly reduce the copy duration. This is done by identifying and transferring the data signatures (instead of the data itself) to the target wherever possible.
To enable DASH Full on a storage policy copy, see Modifying a Storage Policy Copy Deduplication Settings.