This is part 2 of the Full Backups and More blog post. You can see Part 1 here.
An incremental backup strategy also starts with a full backup. After the full backup, incremental backups then back up data that has changed since the last backup. This includes either the last full backup, or the last incremental backup.
As an example, a full/incremental strategy could start with a full backup on Sunday night. On Monday night, an incremental backup would back up all the files that changed since the last full backup. On Tuesday night, the incremental backup would back up all the files that changed since the incremental backup on Monday night. Similarly, the Wednesday night backup would back up all files that changed since the last incremental backup on Tuesday night. This repeats until Sunday when another full backup starts the process again. As the week progresses, the incremental backups stay about the same size.
Restoring a Full/Incremental Backup Set
Assume for a moment that each of the backups was stored on a different tape. If the system crashed on Thursday morning, how many tapes would you need to recover the data?
The answer is four. You would first need to recover the full backup from Sunday. Because the incremental backups would be backing up different data each day of the week, each of the incremental backups must be restored and in the chronological order.
Sometimes, people mistakenly think the last incremental backup would have all the relevant data. Although it might have some relevant data, it doesn’t have everything.
As an example, imagine you worked on a single project file each day of the week, and the system crashed on Thursday morning. In this scenario, the last incremental backup would hold the most recent copy of this file. However, what if you compiled a report every Monday but didn’t touch it again until the following Monday? Only the incremental backup from Monday would include the most recent copy. An incremental backup from Wednesday night or another day of the week wouldn’t include the report.
Choosing Full/Incremental or Full/Differential
A logical question is, “Why are there so many choices for backups?” The answer is that different organizations have different needs.
For example, imagine two organizations perform daily backups to minimize losses. They each do a full backup on Sunday, but are now trying to determine if they should use a full/incremental or a full/differential strategy.
The first organization doesn’t have much time to perform maintenance throughout the week. In this case, the backup administrator needs to minimize the amount of time required to complete backups during the week. An incremental backup only backs up the data that has changed since the last backup. In other words, it includes changes only from a single day. In contrast, a differential backup includes all the changes since the last full backup. Backing up the changes from a single day takes less time than backing up changes from multiple days, so a full/incremental backup is the best choice.
In the second organization, recovery of failed systems is more important. If a failure requires restoring data, they want to minimize the amount of time needed to restore the data. A full/differential is the best choice in this situation because it only requires the restoration of two backups, the full and the most recent differential backup. In contrast, a full/incremental can require the restoration of several different backups, depending on when the failure occurs.
If you have unlimited time and money, the full backup alone provides the fastest recovery time. Full/incremental strategies reduce the amount of time needed to perform backups. Full/differential strategies reduce the amount of time needed to restore backups.
Q. Your backup policy for a database server dictates that the amount of time needed to perform backups should be minimized. Which of the following backup plans would BEST meet this need?
A. Full backups on Sunday and full backups every other day of the week
B. Full backups on Sunday and differential backups every other day of the week
C. Full backups on Sunday and incremental backups every other day of the week
D. Differential backups on Sunday and incremental backups every other day of the week
Answer is C. A full/incremental backup strategy is best with one full backup on one day and incremental backups on the other days.
A full backup every day would require the most time every day.
Differential backups become steadily larger as the week progresses and take more time to back up than incremental backups.
Backups must start with a full backup, so a differential/incremental backup strategy is not possible.