Mirror Backup

A mirror backup is a type of full backup, which means that the entire filesystem or files to be backed up are copied to another media preserving the current filesystem structure. A mirror backup will have identical copies of all backed up files, and will keep them in a “native ordered” fashion.

Mirror backups are commonly performed by users as a “copy operation”, whenever you copy a file or folder from one location to another (within the same storage media or to another), you’re making a mirror backup of that file, since the copied file will be identical to the original one.

However the term “mirror backup” usually refers to full system backups, were an entire filesystem or folder is copied to another media.

The main advantage of mirror backups is that they are readily accessible without any additional software, for an instance, let’s say that you have a folder in your computer that has all your travel photos and you copy that folder to a CD, DVD or USB drive. The resulting copy will be exactly the same as your original one and you can use that copy to restore the original one by just copying and pasting the files from the backup media to the original location.

Mirror backups can be performed without any special software or hardware, so it becomes the cheapest backup method.

However there are some major drawbacks. The time needed to create the first incremental or differential backup is the same as the time needed to create a mirror backup, but the time saving comes with further backups. With mirror backups, each backup will take nearly the same time as the first one, as the entire filesystem is being copied over completely. Another drawback is the amount of storage space needed for each mirror backup, since it will be equal to the space used by the files in their original location. Therefore, mirror backups are useful when it comes to backing up just a few files or when you’re backing up files that won’t be modified for a while and they are being backed up for storage purposes. If you’re looking into keeping updated backups, “roll-back” capability and quick restore access to huge amounts of data, you should look into differential or incremental backups.

The integrity of mirror backups is usually verified using “signing” or “fingerprint” algorithms like MD5 or SHA, which create a unique fingerprint for every file and those fingerprints can easily be compared to the original ones.