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The ways of data backup include full backup, differential backup and incremental backup, so what is the difference between these three backup methods, and how to choose them in specific applications? This paper makes an introduction to these problems.

A brief introduction to the backup method

1. Full backup (Full Backup)

Backing up all selected folders does not depend on the archive properties of the files to determine which files to back up. During the backup process, any existing tags are cleared and each file is marked as backed up. In other words, clear the archive properties.

A full backup is a complete copy of all data or applications at a point in time. In practical application, the whole system is completely backed up with one tape, including the system and all the data. The biggest advantage of this backup method is that you can recover the lost data with only one tape. As a result, the recovery time of the system or data is greatly accelerated. However, its deficiency is that there is a lot of duplicate information in the backup data in each full backup tape; in addition, because the amount of data that needs to be backed up is quite large, the backup takes a long time.

2. Differential backup (Differential Backup)

Back up data that has changed since the last full backup. During a differential backup, only those selected files and folders that are marked are backed up. It does not clear the mark, that is, it is not marked as backed up after the backup. In other words, the archive properties are not cleared.

Differential backup refers to the backup of those files that have been added or modified during the period from a full backup to the differential backup. When we restore, we only need to restore the first full backup and the last differential backup.

For example, on Monday, the network administrator routinely makes a full backup of the system; on Tuesday, it is assumed that there is only one more asset list in the system, so the administrator only needs to back up the asset list; on Wednesday, there is another product catalog in the system, so the administrator has to back up not only this catalog, but also Tuesday's asset list. If there is an extra payroll in the system on Thursday, the content that needs to be backed up on Thursday is: payroll + product catalog + asset list.

Differential backup not only avoids the defects of the other two backup strategies, but also has their own advantages. First of all, it has the advantages of short incremental backup time and saving disk space; secondly, it has the characteristics of less tape needed for full backup recovery and short recovery time. The system administrator only needs two tapes, the full backup tape and the differential backup tape the day before the disaster, to restore the system.

3. Incremental backup (Incremental Backup)

Back up data that has changed since the last backup, including full backup, differential backup, incremental backup. During an incremental backup, only marked selected files and folders are backed up, which clears the markup, that is, marking files after backup, in other words, clearing archive attributes.

Incremental backup means that after a full backup or the last incremental backup, each subsequent backup only needs to back up the files that have been modified compared with the previous one. This means that the objects of the first incremental backup are the files added and modified after the full backup, the objects of the second incremental backup are the files added and modified after the first incremental backup, and so on.

For example, if the system fails on Thursday morning and loses a large amount of data, you need to restore the system to the state it was on Wednesday night. At this time, the administrator needs to first find the full backup tape of Monday for system recovery, and then find Tuesday's tape to restore Tuesday's data, and then find Wednesday's tape to restore Wednesday's data. This is obviously much more troublesome than the first strategy. In addition, the reliability of this backup is also poor. In this kind of backup, the relationship between the tapes is like a chain, one link after another, and a problem with any one of the tapes will cause the whole chain to disconnect.

The most obvious advantage of this backup method is that there is no duplicate backup data, so the amount of backup data is small, and the backup time is very short. But the data recovery of incremental backup is troublesome. You must have the last full backup and all incremental backup tapes (if one of the tapes is lost or damaged, the restore will fail), and they must be restored one by one in the chronological order from the full backup to the incremental backup, so this greatly increases the recovery time.

Second, the difference between differential backup and incremental backup

From the above conceptual analysis, we can see that the difference between differential backup and incremental backup lies in the reference point of their backup: the reference point of the former is the last full backup, differential backup, or incremental backup, and the reference point of the latter is the last full backup.

The following figure shows the difference between differential backup and incremental backup, where Cumulative is differential backup and Differetial incremental backup:

3. Examples of combined applications of different backup types

1. Combination of full backup and differential backup (recommended)

Taking the weekly data backup plan as an example, we can make a full backup on Monday and a differential backup from Tuesday to Friday. If the data is corrupted on Friday, you only need to restore the full backup on Monday and the differential backup on Thursday. This strategy takes more time to back up data, but less time to restore data.

2. Combination of full backup and incremental backup

Take weekly data backup as an example, full backup on Monday and incremental backup from Tuesday to Friday. If the data is corrupted on Friday, you need to restore the normal backup on Monday and all incremental backups from Tuesday to Friday. This strategy takes less time to back up data, but more time to restore data.

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