Incrementals forever

An incremental backup is a backup method where multiple backups are kept (not just the last one). These backups will be incremental if each original piece of backed up information is stored only once, and then successive backups only contain the information that changed since a previous backup.

As a backup method, it is highly efficient, since it allows for the illusion of storage of N copies of size S information chunks, with a total storage requirement much lower than NxS. If the original information that is backed up does not change between backups, the total size will approach just S. If it changes almost completely, the NxS limit will be approached.

Incremental: definition
A “normal” incremental backup will only back up files that have been changed since the last backup of any type. This provides the quickest means of backup, since it only makes copies of files that have not yet been backed up. For instance, following a full backup on Friday, Monday’s tape will contain only those files changed since Friday. Tuesday’s tape contains only those files changed since Monday, and so on. The downside to this is that in order to perform a full restore, one needs to restore the last full backup first, followed by each of the subsequent incremental backups to the present day in the correct order. Should any one of these backup copies be damaged (particularly the full backup), the restore will be incomplete.rrr

An example of a typical incremental backup command in MS-DOS would be: xcopy c:\source\*.* d:\destination\*.* /s /m

Synthetic full backup
A synthetic backup is a form of an incremental backup that is possible when there is a separate computer that manages the backups. The backup server takes a typical incremental backup of the system in question and combines this data with the previous backups to generate a new synthetic backup. This new synthetic backup is indistinguishable from a normal full backup and shares all the advantages, such as faster restore times.

Incrementals forever
This style is similar to the Synthetic backup concept. After an initial full backup, only incremental backups are sent to a centralized backup server. This server keeps track of all the incrementals and sends the proper data back to the server during restores. This can be implemented by sending each incremental directly to tape as it is taken and then refactoring the tapes as necessary. If enough disk space is available, an online mirror can be maintained along with previous incremental changes so that the current or older versions of the systems being backed up can be restored.This is a suitable method in case of banking systems.

Block Level Incremental
This method only backs up the blocks within the file that changed. This requires a higher level of integration between the filesystem and the backup software. Block-based technologies work with heavy changing units (blocks of 8Ks, 4Ks or 1K), they work independently on the file system. At the moment, this technology achieves the highest relative compression of the data, turning into a great advantage for the security copies carried out through the Internet.

-Sylvester

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