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19 Apr 2016

Techie Tuesday: How backup sets are created and maintained

Continuing with our popular Techie Tuesday blog posts, today we cover how backup sets are created and maintained. We will look at how Full, Incremental and Differential images and backups are saved to the target folder specified in your backup definition file. Also, which backup set is chosen when you run a Differential or Incremental.

Backups are grouped into ‘Sets’. A backup set contains first a Full backup and subsequent linked incremental and differential backups. This is sometimes referred to as a Backup Chain. You can see the links by looking at the default file name which is a unique backup set identifier, followed by a number pair.

The above folder contents shows (marked in red) a single backup set {4F6F6CACA58B077D} which contains a full image and 4 increments. The end of the image file contains a number pair? ‘xx-yy.mrimg’

‘xx’ is the increment number.
00 — Full image
01 — first incremental/differential
02 — Second incremental/differential

‘yy’ is the file number and is always sequential. 00, 01, 02, 03… . This will be different to the increment number only if files have been split. This can happen if files larger than 4GB are saved to a FAT32 file system or you are saving an image to multiple DVDs.

So the first file for a full image always ends with ‘00–00.mrimg’.

Note: The Image ID, file and increment numbers are also stored as data inside the files. Renaming a file does not affect the integrity of the set, Macrium Reflects ability to append to the set, or restoration of the files.

How is a set chosen when an Incremental or Differential backup is run?

Each backup set is grouped by similar backup types. This means that a single set will only consist of images of *exactly the same partitions or a File and Folder backups of exactly the same selection criteria.
*Note: A change in partition layout will cause a new backup set to be created even if the drive letters are consistent. The partitions must have the same disk offset and length and must be from the same disk, i.e, the disk must have the same Disk ID.
e.g, If you run an Incremental image of only drive ‘C’ and the target folder contains 4 image files:

6698CD700DF88DF4–00–00.mrimg Drives: C, D, E Created 1st Jan 2015
430D57E2CEEA8552–00–00.mrimg Drive: F Created 2st Dec 2014
1EB1112ABA7C3898–00–00.mrimg Drive: C: Created 1st Nov 2014
D407A9E1BF98D822–00–00.mrimg Drive: C: Created 1st Oct 2014

Then the newly created image file will be 1EB1112ABA7C3898–01–01.mrimg. This is because file 1EB1112ABA7C3898–00–00.mrimg is the most recent full image in the target folder that contains exactly the same partitions as the current Incremental image. The next Incremental (or Differential) image would be 1EB1112ABA7C3898–02–02.mrimg and so on…
Note: If there was no existing backup set that contained only drive ‘C:’ then a new backup set (full) would be created.

Doesn’t this get confusing if multiple backup types are saved to the same folder?

Not if you use the ‘Restore’ tab in Macrium Reflect to view your images to mount and/or restore. The Restore Tab can be restricted to show only images that contain a particular drive and also be sorted by date. So, if you want to restore drive ‘C’ as it was last week than you can easily find it. In fact Backup Sets are an abstraction that you don’t need to worry about when restoring data, they are only relevant to optimizing backup speed and storage space.

You can read the full article on our knowledge base.

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