what's the difference between differential and incremental backups?

Differential and Incremental backups are a way in which you can save space when running backups, particularly if you're using a backup schedule.

The simplest form of backup is a full backup. That will make a copy of everything on your computer. All your files and folders, as well as Windows itself and all the programs and apps you've got installed. In effect, you're copying your entire system onto your backup drive.

That's obviously going to take up a lot of space on your drive. And if you run another full backup the following day, and then another, and another, you're quickly going to run out of space to store all those backup files.

Differential and Incremental backups get around this problem by only recording what has changed since the last backup. What files have been altered or added or deleted, rather than blindly backing up everything.

Differential backups always refer back to the last full backup. They compare your system as it is right now with what's included in the full backup.

Incremental backups, on the other hand, refer back to the last backup taken, whether that was an Incremental backup or a full backup. Each Incremental backup builds on the previous backup.

In this guide, we'll look at the difference between Differential & Incremental backups and compare them against each other.

Home » Computer Guides » The Difference Between Differential and Incremental Backups.

the difference between how differential & incremental backups work.

The easiest way to see the difference between Differential & Incremental backups is to see how they work in practice.

Below we've got a very simple step by step guide to how each type of backup actually works.

Then later we'll look at the strengths and weaknesses of each.

how differential backups work.

So we make our first full system backup.

That creates a file on your backup drive. The file will contain everything on your computer.

It'll obviously be a large file.

We'll call it 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, we'll call it Pic 1.

With Pic 1 saved on the PC, we run a Differential backup.

The Differential will check what's on the computer against what's already backup in the Full Backup.

And obviously, it'll discover that a new file (Pic 1) has been added.

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

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 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.

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 been backed up in the Full Backup.

It discovers that 2 new pictures have been added (Pics 1 & 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.

Now that raises a problem, because our first picture, Pic 1, has been backed up twice, even though it hasn't changed in any way, it has still been backed up again.

We'll do it one more time. We'll add a picture, Pic 3, and then run another Differential backup.

And you guessed it.

The Differential backup only checked the Full Backup, discovered that 3 pictures (Pics 1, 2 and 3) had been added, and so backed those up.

And now you can see the big issue with Differential backups. Duplication. Pic 1 has been backed up 3 times and Pic 2 twice. The more we run Differential backups, more and more files will be copied over and over again.

And because of that, the Differential backup file sizes are getting larger and larger.

If left unchecked, Differential backups can quickly become larger than a full backup would be.

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 we'll call Full Backup.

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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.

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.

With Incremental backups, files aren't duplicated within the backups.

Once a file (whatever it may be) has been included in a backup, be that a full backup or Incremental backup, then that's it. The file won't get backed up again unless it changes in some way.

By doing this, Incremental backup file sizes remain relatively small.

I think you've got the idea now, but 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 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.

differential & incremental file sizes.

In our simple example above, it's easy to 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.

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.

But that's not the end of the story...

differential vs incremental backups.

Differential Vs Incremental backups - Round 2 - The Differential strikes back.

Differential backups can actually offer you more protection than Incrementals.

As an example of how this might work, we'll use the backup sets we "created" above to restore our computer and our 3 pictures.

But here's the thing, the spanner in the works, the second Differential (Diff 2) and the second Incremental (Increm 2) backups don't work. They'll fail to complete. There's something wrong with both of them, maybe they've been corrupted on the backup drive.

The point is that neither Diff 2 nor Increm 2 will work when we try to restore our computer.

restoring PC from differential backup set.

We'll start by using the Differential backup set.

The backup software will call upon the last full backup made.

That'll restore Windows, all our programs and apps and get our system working again.

Great, now we need to get our picture files back.

So now the backup software will go to the latest Differential backup (Diff 3).

It completely misses out Diff 1 and Diff 2 because everything that's inside those two backups is also contained in Diff 3.

In our example, we want the 3 pictures (Pics 1, 2 and 3) back. And they're all inside the third Differential backup, Diff 3.

It's all we need to get up and running again.

And this is an important point about Differential backups. Duplication. The very thing that is regarded as a weakness is also a strength of Differentials.

In our little example, we as the user would be blissfully unaware that there was a problem with the second Differential backup (Diff 2).

All we'd see is our computer working properly again and our 3 pictures files returned.


restoring PC from incremental backup set.

So now we'll run the same restoration, but this time using our Incremental backup set.

The backup software will call upon the last full backup made.

That'll restore Windows, all our programs and apps and get our system working again.

Great, now we need to get our picture files back.

Then to bring our computer right up to date with all our files, the software begins using the Incremental backups.

Incremental backups have to be used in order.

So it'll start with Increm 1, which will restore our first picture file.

At this point, if I was doing this for real on a clients computer, I'd be thinking "Ho, ho, ho, it's all going so well, I'll be done in an hour, get those golf clubs ready".

But I think you can probably see what's going to happen.

When the backup software tries to load the second Incremental backup (Increm 2), it fails.

And the whole restoration process grinds to a halt.

"There has been a critical error"

Incremental backups are a series of steps, each builds on the last. Rather like links in a chain, they all rely on the previous one.

In this restoration, we'd get our computer working again, which is good, but we'd only get 1 picture file back.

When we're comparing Differential Vs Incremental backups, they both have there own strengths and weaknesses.

Incremental backups are far more space-conscious backups. There isn't any duplication of files.

Differential backups can easily become space hogs, eating up all available capacity on your backup drive.

On the other hand, because of that duplication of files, Differential backups have the ability to recover more of your data if things go wrong.

about backup sets.

Whatever backup software you're using, you'll almost certainly come across the phrase "backup set".

A Backup set begins with a Full Backup. The full backup is the foundation stone on which the backup set is built.

All the following Differential and/or Incremental backups that follow, build upon that full backup. They all "belong" to that full backup.

A new backup set begins when you create the next full backup.

differential or incremental backups?

I can't really say from here which type of backup you should use to supplement your full backups.

So much will depend on how much data you're backing up, how often it changes, how important those changes are to you and of course, how much space you've got on your backup drive.

But at least you can now visualise the differences in how Differential & Incremental backups work. And it's also important to remember how that would affect them if you ever need to use them to recover your computer or files.

the grandfather-father-son backup template.

Most if not all backup software includes 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.


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