Full backup

A full backup is backing up all files in a given storage area, regardless of their state (such as an archive bit setting), last change date, or creation date.

Incremental backup

An incremental backup means backing up only the files that have been modified since the last backup, whether the last backup was a full or incremental backup. One advantage of incremental backups is that they store the least amount of data, because only the newly-modified files are backed up. The disadvantage is that if a restore is necessary, files must be restored first from the most recent full backup, then sequentially from each subsequent incremental backups. In this type of restore operation, files that had been deleted since the full backup are often also restored, because the deletion information is often not written to the backup media.

Differential backup

A differential backup (or delta backup), means backing up only the files that have been modified since the last full backup. A differential backup is very similar to an incremental backup, but there are differences. Incremental backups are used to save tape or disk space, but may or may not reduce the time to do a restore. When recovery is needed, the full backup and all incremental backups are required to fully recover the data. Differential backups do not save tape or disk space. Differential backups that are made subsequent to the first differential backup are bigger than incremental backups that are made at the same time. The restoration time is usually faster when using differential backups as compared to incremental backups. If restoration is necessary, only the full backup

Synthetic backup

A synthetic backup means combining a full backup with subsequent incremental backups or with a subsequent differential backup offline. A full backup, combined with subsequent incremental or differential backups, can be indistinguishable from a separate full backup that is done at the time of the most recent incremental or differential backup. For this to be the case, the subsequent partial backups must include information noting which files have been deleted since the full backup. This typically requires the use of a backup server with a database of information about the files that have been backed up. It can then combine the information from a full backup with that from the incremental or differential backup, adding new and changed files and removing any deleted files, in presenting the contents of the synthetic backup.

Backup and restore window

A backup and restore window is the amount of time a system administrator has been allocated to back up a system. Best practices for system backup dictate that all components be stopped so that a static environment for the backup can be maintained. During this time, applications are unavailable to users. Likewise, if the system must be restored to a previous good state, the system must again be stopped and become unavailable to users.

As system size grows, backup and restore windows might also grow. Therefore, system administrators must manage expectations among both users and management. System administrators must find a balance between speed of backup and restore, and the cost of the hardware and software that are required to meet stated goals.

FlashCopy

Modern storage system provide a feature such as FlashCopy for creating a point-in-time copy of production data for transfer from disk to tape. FlashCopy can also support consistency groups. These consistency groups can be used to create consistent, point-in-time copies across multiple storage subsystems. Because FlashCopy can perform these actions in a very short period of time, the backup can be performed in a very narrow window. The entire process can potentially be no longer than the time necessary for bringing down the system, executing the nearly instantaneous FlashCopy, and then restarting the system. The actual full, incremental, or differential backup is made after restarting the system, effectively masking most of the backup time. The FlashCopy approach eliminates most of the backup time from the backup window.