Incremental Backups
Note: Although you cannot make an incremental backup using Image for Windows, we include information on incremental backups so that you can understand how they work.
Incremental backups include only data that has changed since the most recent backup was performed—whether the most recent backup was a full backup or a previous incremental backup. To use this backup method, you perform a full backup at an interval of your choice—say every two weeks. In between full backups, you perform incremental backups. If you need to restore your entire system, you need to restore the latest full backup followed by each of the incremental backups you performed since that full backup.
For example, suppose that you are relying on file-based backups, and you perform a full backup that includes FILE1, FILE2, and FILE3. Then, you change FILE2, and you perform an incremental backup. This incremental backup will include only parts of FILE2, since you did not change the other files in the most recent full backup. Then, if you change FILE3 and add FILE4 and make another incremental backup, the latest incremental backup will include only data from FILE3 and FILE4.
If you are relying on sector-based backups, you perform a full backup at an interval of your choice and, in between, you perform incremental backups. But, an incremental sector-based backup is not based on files that have been added or changed. Instead, an incremental sector-based backup looks for and includes newly allocated sectors and changes to the contents of any sector since the last backup. Suppose that you move a file without changing its contents. In a sector-based backup, the sector reallocation caused by moving the file is a change that will be included in the next incremental backup, even though you didn’t change the file itself.
Note: Although defragmenting the file system does not change file content, it can lead to many sector-level changes, because defragmenting files moves them from one disk location to another.
Incremental backups are hard to properly manage and tend to be troublesome during disaster recovery. It is not uncommon to discover, while trying to recover from a disaster, that an incremental backup is either lost or damaged, making all subsequent incremental backups worthless. In addition, if you accidentally restore incremental backups in the wrong order, the problems you experience may not manifest themselves until some future date, at which point recovery can become almost impossible.
Differential Backups
Differential backups include only data that has changed since the most recent full backup was performed. To use this method, you make a full backup at an interval of your choice. In between full backups, you perform differential backups, which include all data that has changed since the last full backup. If you need to restore your entire system, you need to restore the latest full backup followed by the latest differential backup. Unlike incremental backups, which rely on every other incremental backup in the chain, a differential backup relies only on the full backup.
For example, suppose that you are relying on file-based backups and you perform a full backup that includes FILE1, FILE2, and FILE3. Then, you change FILE2, and you perform a differential backup. This differential backup will include only parts of FILE2, since the other files in the most recent full backup have not changed. If you then change FILE3 and perform another differential backup, this differential backup will include both data from FILE2 and FILE3.
A differential sector-based backup includes any sector that has changed or been allocated since you created the last full backup.
Differential backups are easy to manage during disaster recovery because you need to restore only the last full backup followed by the last differential backup. You don’t run as much risk of discovering that a backup is damaged or missing, and since you only need to restore two backups, you are not as likely to restore them in the wrong order.
By its nature, the size of a differential backup grows over time. If you wait long enough between full backups, your differential backup could become almost as large as a full backup, and take almost as much time to create |
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