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What’s an Incremental Backup? – Ask Leo!


Save space by backing up only certain things.

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I have a mental ‘block’ on backing up which unnerves my approach to it. I have managed one full backup (32gb) on an external hard drive. I have just read your article on maintenance but do not understand what is meant by “incremental backup”. Does it simply ‘update’ or overwrite the existing backup or does it create something else that only contains whatever is new since the previous backup? I think the latter is what I would prefer.

What you prefer is, indeed, what it is.

There are three different types of backups: full, incremental, and differential. It’s important to understand which is which and how they’re used to make sure you’re backing up appropriately, but not using excessive disk space.




A full backup contains a copy of everything. An incremental backup contains a copy of only those things that changed since the previous backup. A differential backup contains a copy of only those things that changed since the most recent full backup. Unless you know otherwise, monthly full plus daily incremental backups are a good place to start.

Full backup

A full backup is a backup of everything.

A “full backup of C:”, for example, would be a backup of all the files and folders and system information on the C: drive. A full backup of “your system” might include the entire contents of all the drives connected to your system.

A full backup stands alone. It’s a snapshot of your machine at a point in time. You can then restore the entire full backup to return your machine to the state it was in when it was taken. Since it has everything, you can also use it to restore individual files or folders as needed.

Pro: a full backup stands alone. It has everything. There’s no guessing about what should or should not be backed up.

Con: a full backup can be big, sometimes very big. It has everything.

Incremental backup

An incremental backup includes only those things that changed since the previous backup.

After starting with a full backup, an incremental backup backs up only those files changed since the full backup was taken. The next incremental backs up only those files changed since the previous incremental, and so on.

Most files on your machine rarely change. That means an incremental backup is much smaller. For example, on my desktop machine, a full backup is roughly 300 gigabytes in size. Incremental backups range between 4 and 10 gigabytes, depending on how much happened on the machine that day.

Each incremental backup relies on the backup that preceded it. In order to restore a machine, the starting full backup plus all the incremental backups thereafter must be available.

My machine performs a full backup on the first of each month and then a daily incremental. To restore my machine to the backup taken on the 10th, I need the initial full backup taken on the first, plus the nine incremental backups to reach the 10th day. Since something unique could have changed each day, the restore processes the full backup, then each incremental in turn, until it has processed them all.

Most backup software makes this easy. Typically, you point the software at the collection of backups — both full and incremental — and tell it, “I want to restore to this date”, and the complexity is managed for you.

Pro: compared to an equivalent set of full backups, incrementals use significantly less disk space for storage.

Con: the baseline full backup, plus all the incremental backups, must be preserved and available in order to restore.








Differential backups

Differential backups are a kind of hybrid. They’re incremental backups with a fixed starting point. Rather than backing up only the changes from the previous backup, each differential backup includes all the changes from the starting backup. So day 2’s backup would include changes from day 1 to day 2. Day 3’s backup would include changes from day 1 to day 3.

Pro: You can restore from only two backups: the initial full one plus that day’s differential.

Con: Differentials grow in size from day to day. Differential backups end up being larger than incremental backups, but not as large as taking a full backup every day.

Backups in action

Let’s compare using these three methods to back up a “C:” drive. This table shows what each type of backup would include on a series of successive days.

Day Full Backup
Contents
Incremental Backup
Contents
Differential Backup
Contents
1 Entire Drive Entire Drive Entire Drive
2 Entire Drive All changes since Day 1 All changes since Day 1
3 Entire Drive All changes since Day 2 All changes since Day 1
4 Entire Drive All changes since Day 3 All changes since Day 1
5 Entire Drive All changes since Day 4 All changes since Day 1
Entire Drive All changes since the day before All changes since Day 1

Now let’s look at restoring data. This table shows which backups must be available in order to restore to a specific day.

Restore to Day Full Incremental Differential
1 Full backup from Day 1 Full backup from Day 1 Full backup from Day 1
2 Full backup from Day 2 Full backup from Day 1
+ Day 2’s incremental
Full backup from Day 1
+ Day 2’s differential
3 Full backup from Day 3 Full backup from Day 1
+ Day 2’s incremental
+ Day 3’s incremental
Full backup from Day 1
+ Day 3’s differential
4 Full backup from Day 4 Full backup from Day 1
+ Day 2’s incremental
+ Day 3’s incremental
+ Day 4’s incremental
Full backup from Day 1
+ Day 4’s differential
5 Full backup from Day 5 Full backup from Day 1
+ Day 2’s incremental
+ Day 3’s incremental
+ Day 4’s incremental
+ Day 5’s incremental
Full backup from Day 1
+ Day 5’s differential
Full backup from Day … Full backup from Day 1
+ all incrementals to Day …
Full backup from Day 1
+ Day … differential

What type of backup to use

The choice of which type of backup to use can be daunting. Daily full backups seem the simplest, but you’ll quickly find yourself running out of disk space. Incrementals seem like a great alternative, but you may find yourself in backup-management hell as you try to keep track of which files you need to keep for how long. Differentials seem like an interesting idea, but are they really all that different than incrementals?

Here’s my suggestion:

  • Periodic full backups. I recommend a full backup once a month. Save these for “a while” so if you need to find something from, say, three months ago, you may be able to recover it.
  • More frequent incremental backups. I recommend a nightly incremental backup. As the month goes on, you’ll accumulate a collection of incremental backups in addition to the full backup at the start of the month. You’ll be able to revert or recover from any day in the month. Once a month, “clean the slate” by removing the previous month’s incremental backups as a new full backup is taken.

Why not just do a nightly full backup and allow it to overwrite the previous day’s backup?

You could. The problem is you’re assuming you’ll always detect a problem or a missing file within a day, before the backup containing it is overwritten.

Experience shows that things rarely work that cleanly. It’s not at all uncommon to wonder days, or even weeks, later, “Where did that file go?” Being able to search your daily backups for a file can be a real lifesaver.

Looking at it another way

Since I know that these concepts are often confusing and sometimes difficult to grasp, I’d like to also point you to this article: What Backup Type Do I Want: Full, Incremental, or Differential? It covers essentially the same topic using a slightly different visual representation.

Above all, back up!

Don’t let all this intimidate you into not backing up at all! Most backup programs make these choices fairly easy and do a good job of managing files for you and completely automating the process.

Computers are good at that.

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This post was written by Leo Notenboom and was first posted to AskLeo.com



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