We will be looking at Macrium's Synthetic Backup and Consolidation functions to find out how the settings impact actual real world backups.

Synthetic backups refer to the creation of a full backup - instead of physically creating the backup of the source the software will artificially combine and consolidate incremental backups into a full backup. This will reduce the incremental chain length and give you a new baseline full image. Synthetic backups and consolidation have some advantages:

  • Saves Time - Synthetic backups are generally quicker to perform than an actual full backup*
  • Save Storage Space - Creating full synthetic backups reduce storage requirements (merging an incremental into a full will not increase the full by the exact size of the incremental)

*In most cases the time taken for a full synthetic backup is less than taking a manual full. The I/O requirements for a manual backup generally exceeds the requirements of a synthetic backup. Conversely we have observed some cases where the synthetic takes longer and almost as long as a manual full. It's worthwhile to monitor the merging times vs manual fulls to optimize your backup strategy.


For our first scenario we will be using Macrium to run a full backup image with 5 incremental backups. We will show the implications and considerations of using a fixed length incremental chain. The base image is taken of a server with a used hard drive size of 80GB. The resulting full backup on medium compression came to 51GB.

The settings we used to make backup are as in the screenshot below:

Notice the amount of incrementals to keep is set at 5. When the 6th backup is run the following happens: incremental 2 will be merged into incremental 1 forming a new incremental 1(but named 02-02 in explorer). A new 5th incremental will join the end of the chain. Running Backup 7 will merge incremental 3 into the newly formed incremental 1 (02-02), once again a new incremental added to the end.

Visually, this is what happens, illustrating the 6th and 7th incremental backup:

Here is what the backup folder would look like after the 6th backup and first consolidation:

Notice how the first incremental 01-01 is  gone and replaced by 02-02. The base image remains untouched. The first incremental (into which all others will merge) grows linearly according to each individual incremental size.

This is an effective backup plan when you need to have a reference base image with a set incremental chain length which would correspond to your required recovery targets (how far back would you need to be able to recover, in this case 5 days from the last backup or the first day of the original backup). Aspects to take note of:

  1. The base image remains anchored in time, its creation date does not change and remains a reference point if you need to restore data from the original image.
  2. The first incremental grows in linearly in proportion to the individual incremental sizes when they merge.
  3. At some stage the first incremental will exceed the size of the base image.
This what the folder looks like after the 7th backup:

Note that the first incremental (03-03) has now grown in size in proportion to the 2 incrementals that merged to form it.

A situation to consider is when the merged incremental nears the size of the base image. This situation is less than ideal because dealing with such a large incremental when doing a full restore becomes cumbersome and will increase the restore time.

At this stage you can consider doing a Synthetic Full backup. You can enable this by ticking the "Create a Synthetic Full If possible" option:

In this case the first incremental will merge into the base image. The synthetic full took only a few minutes, much faster than performing a normal full.

Here is a visual representation:

This is what the folder looks like afterwards:

The large (400mb) first incremental from before is merged into the base image freeing up disk space. The base image size only increases slightly (less than 400MB) due to data inside the image being overwritten and not appended.

As a result of the synthetic backup the base image date has now moved forward in time. Subsequent backups will keep moving the base image forward in time.

Differential Backups

Next we'll look at how differential backups affect the incremental queue.

We will keep the backup definition as is but add a differential at the end.

Next we run run 5 incrementals. This will merge all the initial 5 incrementals into the base image and create 5 new ones after the differential. The differential will now be directly after the base image:

Notable points:

  • The 5 new incrementals are now dependent on the differential
  • You will not be able to do a Synthetic Full anymore - the differential can't merge into the base images
  • All subsequent incrementals will be merged and consolidated into each other but not the differential
  • Differentials act as 'checkpoints' that can't be moved unless deleted (note that the incremental chain that's dependent on the differential will also be deleted)

In summary, Macrium's synthetic backups and consolidation gives you immense flexibility when designing your backup regime. Backup plans are often a balance between storage space and the required restore history so this may help in optimizing your strategy.

Extra Reading :

Macrium has a standalone consolidation application that can be used to merge backups independently. This useful when archiving backups or when managing backups away from the backup source.