Home » The Difference Between Differential & Incremental Backups.

The Difference Between Differential & Incremental Backups.

Most third-party backup software (such as Acronis, Macrium Reflect, Easeus Todo etc) will allow for the creation of different types of backups, namely, full, differential and incremental backups. Whilst the full backup requires no real explanation (it does what it says on the tin), differential and incremental backups, however, can be confusing.

The Differences Between Full, Incremental And Differential Backups.

  • Full Backup – Creates a complete copy of everything that has been included in the backup. Full backups use the most space on your backup device.
  • Differential Backup – Creates copies of everything that has changed since the last full backup. Differential backups begin as very small backups (tiny when compared to the full backup that they’re based on), but over time they get larger and larger and can even become larger than the original full backup.
  • Incremental Backup – Creates copies of all the changes since the last full or incremental backup. Incremental backups are the smallest backups and will remain so over time.

Why Do We Have 3 Different Methods Of Backing Up?

The short answer is to save space on the backup drive. In an ideal world, we’d have limitless space available and so could simply create full backups over and over again. But generally speaking, most of us don’t have that much space available. Whatever capacity drive you’re using will quickly fill up if you’re running full backups every day.

Both differential and incremental backups are designed to save space. They do this by only backing up what needs to be backed up. By only saving the files that have been added, changed or altered in some way.

However, the way in which they work is different and each has its own strengths and weaknesses.


The Difference Between How Differential & Incremental Backups Work.

Both differential and incremental backups require a full backup to start with. Then as differential or incremental backups are created, they are each built upon the full backup.


How Differential Backups Work.

To begin with, we’ll need to take a full backup, which will necessarily create a very large file on the backup device.

Diagram representing a full backup.

Since both differential and incremental backups only record changes on your computer, we need to change the files on the computer in some way.

We’ll add a picture. We’ll simply save a picture file into the Pictures folder, for simplicity we’ll call it Pic 1.

We could just run a full backup again. But that would create another very large file when all we really need is to add the picture file (Pic1) to the backup.

Which is where the differential backup comes in.

The differential will check what’s on the computer against what’s already saved in the Full Backup.

And obviously, it’ll discover that nothing has changed except for the addition of a new file (Pic 1).

That’s the only thing that has changed. So it’ll only back up the picture file Pic 1.

Diagram representing a full backup and a differential backup.

Quick Note

Now you could be forgiven for thinking that the differential backup would simply add the picture file (Pic 1) to the full backup. But it doesn’t do that.

What it does is to create a second backup file on your backup drive. This second file will only contain (in our simple example) Pic 1 because that’s the only thing that has changed since the full backup was taken. Which means that it’ll be incredibly small in size.


Creating More Differential Backups.

So far so good. Now we’ll add a second picture, Pic2, and run a second differential backup.

The differential backup checks what’s on the computer against what has already been backed up in the full backup.

It discovers that 2 new pictures have been added (Pic 1 & Pic 2), and so creates a new backup file that contains both Pic 1 and Pic 2.

Notice that the differential backup DID NOT check what was in the previous differential backup (Diff 1). It totally ignored Diff 1.

Differential backups only ever refer back to the last full backup.

The backups are getting larger

To emphasize the point, we’ll do it one more time. We’ll add another picture, Pic 3, and then run another differential backup.

And again the differential checks what’s on the computer against what is already saved in the full backup.

When it does that, it’ll obviously discover 3 new pictures have been added to the machine since the full backup was made.

Yet again it totally ignores what has already been included in previous differential backups.


How Incremental Backups Work.

Now we’ll do exactly the same thing, adding pictures one by one, only this time we’ll run Incremental backups.

So we start with a full backup, just as we did earlier.

Which again will be a very large file.

Diagram representing a full backup.

And just as we did earlier, we’ll add a picture, Pic 1 and then run the first Incremental backup.

The Incremental backup will check what’s on the computer against what’s already in the full backup.

It’ll discover Pic 1 and will create a new backup file that contains just Pic 1.

So far it’s exactly the same as using Differentials. There’s no difference between them at this point.

It’s when we do another backup that things get interesting.

Diagram of a full backup and a single incremental backup

So we add another picture, Pic 2 and run the second Incremental backup.

The Incremental backup then checks what’s on the computer against what’s already backed up in BOTH the Full Backup and the previous Incremental backup (Increm 1).

It checks them both.

By doing that, it discovers that ONLY one picture (Pic 2) has been added to the computer since the last backup was run.

So it creates a new backup file that only contains Pic 2.

The backups are roughly the same size

No Duplication of Files.

With incremental backups, files aren’t duplicated within the backup files. Once a file (whatever it may be) has been included in a backup, be that a full or incremental backup, then that’s it. The file won’t get backed up again unless it changes in some way.

By working this way, incremental backup file sizes can remain relatively small. Whereas differential backups grow and grow the further away from the full backup they get.

And that is the main difference between differential and incremental backups


We need to finish the example we started by adding a third picture (Pic 3) and running the third incremental backup because we’ll use this later to demonstrate another important difference between differential and incremental backups.

The incremental backup checks back through all the previous backups and realises that the new picture file (Pic 3) has been added.

It then creates a new backup file with only Pic 3 in it.

Full backup and 3 incrementals

Differential & Incremental File Sizes.

In our simple example above, it’s easy to see how the size of differential backup files can grow quickly. Over time they can become huge.

Whereas incremental file sizes remain much smaller. You could think of them as being “smart” backups, in that they only backup what needs to be backed up. there isn’t any duplication.

And since both types of backup are supposed to be space-saving backups, we have a clear winner. If space is very tight on your backup drive, then incrementals are the way to go.

So now you may be asking why do we even have differential backups at all. Well that’s not the end of the story..


Differential Vs Incremental Backups.

The whole purpose of creating any backup is to protect you, the user, from data loss. In short, to save your files, or as many of them as possible, should something go wrong with your computer.

With that said, you have to accept that backup files, be they full, differential or incremental backups, are in themselves simply computer files stored on computer devices.

As such, they are as susceptible to damage or loss as any other computer file. And this is a problem. Your carefully created backup plan might not actually work when you need it. When everything has gone wrong with your PC and you decide to restore it using your backup files, they aren’t guaranteed to work.


Restoring Your Computer From Backups.

When it comes to restoring your computer from your backups, then differential backups are by far safer than incrementals. That’s because each subsequent differential contains all the data that was in the previous differential backups.

To illustrate, let’s assume that the second differential backup file has become corrupted in some way and is now unusable (unreadable).

If we try to restore our computer, we’d first use the full backup, and then update it using just the final differential backup (Diff 3).

We don’t need Diff 1 and Diff 2 because all the data in those backups (in our example Pic 1 and Pic 2) is contained in Diff3.

Using a differential backup set to restore a computer.

But restoring a PC from an incremental backup set requires every single incremental backup to work.

If just one of the incremental backups becomes corrupted, then the whole process will halt.

You can go so far, but then no more. You can’t move on to the next incremental backup until the current one completes.

So in our example, we could get the computer up and running from the full backup.

Then restore Pic 1 from the first incremental backup, but now it will stop. That’s as far as we can go because the second incremental backup (increm 2) doesn’t work.

Restoring from incremental backup sets

Space Saving Vs Protecting Your Files.

That’s why we still use differential backups. It’s why nearly all backup software offers it at least as an option. They are simply safer. You can lose one or two differential backups and still restore your computer. But the same cannot be said of incrementals.

And so in your backup plans, you have to consider the space-saving efficiency of incrementals against the robustness of differentials.


The Grandfather-Father-Son Backup Template.

Most if not all backup software include what are generally referred to as backup templates. These templates are predefined schedules for how the backups will run on your computer.

One of the best known of these templates is called the Grandfather, Father, Son template.

Generally, it will work like this, but different software may have variations on it. You take a Full Backup each month, that’s the Grandfather. Then Differential backups weekly, they’re the Father. And Incremental backups daily, the Son.

By working like this, you create a backup set that will combine the robustness of Differential backups with the space efficiency of Incrementals.

So taking a month as being 4 weeks (28 days), the backup set will have

1 Full Backup
4 Differential backups
28 Incremental backups.
And then it starts over with another backup set the following month.


Conclusion.

When considering how to set up your backup plans and schedules, it’s important to note the differences between using differential and incremental backups, because it often comes down to being a compromise between how much space you’ve got available and how important your data is.

Remember that, in general, differentials are safer than incrementals but at the cost of space.

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