Image and clone are two very confusing backup terms.
One reason is a deep, dark, dirty little secret of the industry: we don’t all agree on what these two terms mean. Quite literally, one person’s clone is another person’s image, and vice versa.
It gets confusing, and if you’re hearing different things from different people, that’s why.
I’ll provide what I believe are the most accurate and common definitions. Hopefully, that means you’re more likely to hear people agree than disagree with what I’m saying.
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A clone is a bit-for-bit identical copy of a drive1.
To understand what that means, we need to review what’s typically on your computer’s hard disk drive. Content falls into three broad categories.
Another important thing to know is that data isn’t kept together on a hard drive. Particularly as you use it over time, data gets spread out over the disk, and when you delete a file, the area it occupied is marked as available (or free, or empty). The net result is that over time, portions (or “fragments”) of files are spread out over the entire hard disk, with free space between.
A clone is an exact duplicate of the original. Not only is all the overhead and data copied, but the content on the disk marked as free space is also copied. The exact layout and organization of all the data and free space is preserved.
The most common application of cloning is to make an identical copy of a physical hard drive and put it on a different physical hard drive. An application of this might be to have two drives in one machine. In fact, some people use this as a backup strategy; they have two identical drives in their machine, and periodically clone the drive they use to the backup drive. Essentially, they take a snapshot of their working drive. If they ever need to restore or replace the primary drive, they simply switch the cables so the backup drive becomes their working drive.
It is possible to clone a disk to a file. That file can then be used to restore that disk to its bit-for-bit identical state. A file containing a clone will generally be proportional in size to the original hard disk size (not accounting for any possible compression used to create the file).
An image is a copy of all the information on a drive.
Like a clone, an image copies all of the overhead and data stored on a drive. Unlike a clone, an image does not copy free space, and it makes no attempt to preserve the physical layout. This doesn’t affect what you see in terms of where and how files appear as you use a drive; it only affects the physical location of the files on the disk media.
An image is typically made to a file, rather than to another drive, so it’s often called an image file. A complete image of a hard drive still includes boot and partition information. In fact, when you fire up a tool like Macrium Reflect, one of the first things it does is display all the partitions on the hard drive. There are several of them, and if you say, “Back up all the partitions required for reinstalling Windows”, it selects several of them for you. That’s the partition information that gets stored in the resulting image file.
But this time, data and only data gets placed into the image. It includes all of the boot, partition, and overhead information, with your files and folders, and compiles it all into an image file. Since it doesn’t contain any of the drive marked as free space, an image is typically much smaller than the actual size of the drive you’re backing up; it’s proportional to how much disk space has actually been used.
A full-image backup has two main differences from a clone:
- Free space is ignored.
- The layout of the files on the disk is ignored.
Typically, an imaging program copies one file at a time. It looks for a file on C:, copies it to the image file, and moves on to the next file. That process removes any fragmentation present on the original drive. The net result is that the back-up file represents a completely defragmented image. In other words, all of the files are sitting next to each other, perfectly contiguous. When you restore an image file to an empty hard drive, all the files come back perfectly defragmented (in fact, that is one way to defragment a drive completely: back it up to an image and immediately restore it).
Image and clone: which do you want?
Now that we understand the difference, which one do you want to use?
My position is that for backing up your computer, you want an image. There’s no real added value from taking the extra time, space, and resources to create a clone. An image file ends up being smaller and significantly faster to create — and to restore, when the time comes.
When in doubt, choose image.
This article is taken from What Those Words Mean, an overview of several of the most common and confusing terms used relating to backing up.