File Backup Terminology, Part 1 of 2

Over the years various backup technologies have been developed in an attempt to minimize the amount of space required to store backup files, and to reduce the bandwidth required to transfer those files to remote locations.  When faced with the different backup methods that many programs offer, it is easy to become confused, since the terminology used is often not very clear, and it is hard to know the benefits or drawbacks of any one technology. This is not, by far, an exhaustive glossary of backup terms.

Full Backups

This is just what it sounds like.  This is a complete backup of all the data that a user selects when configuring a backup job. The copied files are usually placed into a single file archive and compressed to help save space. Every time another full backup is made, all the files in the source are once again copied an archive. The problem is that often there are only a few new or changed files, and continuously making full backups will end up copying a lot of extra files that don’t really need to be backed up again. This ends up using a lot of extra storage and wastes time. You can of course delete older backups to free up space, but the time is still lost. The extra wear on hard disks or the amount of bandwidth that is used to make frequent full backups must be considered too.

It is a much better idea to make a full backup once in a while, and then figure out a way to only copy the new or changed files on a more frequent basis.

Benefits and Disadvantages of Full Backups

  • Faster restore of all files — When a full restore is necessary full backups are quick because you are only dealing with one archive file.
  • Full backups are large and time-consuming to make — They are not well suited for regular backups such as those performed hourly or daily.

Differential Backups

After creating a full backup archive this backup method helps to reduce the size of subsequent backups by doing a “differential” comparison of the original files and the last full backup. All new and modified files are copied to an archive alongside the full backup.

The important thing to understand is that differential backups are cumulative. Each differential backup backs up everything that is different since the last full backup even if those files are already included in a previous differential. Since Differentials back up only new or changed files, they are a faster backup method than creating a full backup each time. Differential backups are well suited for daily or less frequent backup strategies.

Benefits and Disadvantages of Differential Backups

  • Faster to restore that some other methods — To do a full restore of all backup files, you only need the full backup and the last diff backup.
  • Differential Backups are more demanding on storage than some of the other backup methods, because of data redundancy.
    Each subsequent differential grows significantly until it becomes necessary to create a new full backup.  Then the process starts over.

Incremental Backups

This backup method works similarly to differential backups, but with one important difference that deals with the high level of data redundancy in differentials. Each incremental contains only the files that were created or modified since the last full backup or last incremental.  Incrementals, while not containing as much redundant data as differentials, are still cumulative since successive backups will still contain any files that were already backed but have been modified in some way. Incremental backups are a good solution for more frequent backups such as those performed on an hourly basis.

Benefits and Disadvantages of Incremental Backups

  • Incremental backups can be completed more quickly that differential backups because there is less redundant data being copied.
  • Incremental backups are smaller than differential backups.
  • The number of successive incrementals that can be made between full backups, while still remaining manageable, is much greater than with differentials.
  • Incremental backups may take considerably longer to do complete restores than differential backups because all the individual archives must be merged together one by one with the full backup.

It should be noted that a restore from an incremental backup may fail if one of the sequential backups were to be lost or damaged. Although in all the backups up to the damaged one should be recoverable.

Mirror Backups

Most backup programs will list mirror backups as an alternative to full, differential, or incremental backups, etc.  Some programs use an alternate term for mirrors, such as “simple copy.”   Mirror backups are basically the simplest type of backup.  There are no real backup technologies being employed when making a mirror style backup, only copy technology.  If you copy and paste a folder from one drive to another you have created a mirror backup of that folder.  The mirrored files generally exist in the same state they did in the source, not compressed into archives like with a full backup.  (Although some programs support compressing each file individually and adding encryption)

When to Use Mirrored Backups

Mirror style backups without compressions are good to use when you are backing up a lot of files with compression already applied them.  For example, music files in mp3 or wma format, images in jpg or png format, videos in dvix, mov, or flv format, and most program install or setup files are already compressed.  If you include these files in a normal backup that applies compression you will often notice it will be very slow, and you will gain very little extra compression by doing so. It is best to set up separate backup jobs for compressed files and non compressed files. If your backup program supports include and exclude filters they can be used to either automatically select or deselect the compressed files respectively.

Benefits and Disadvantages of Mirror Backups

  • Mirror backups are much faster when working with compressed files.
  • Because mirrored files are not placed in single archive files there is less concern about corruption.
  • Since mirror backups generally don’t use compression they can require large amounts of storage space, unless other techniques such as hard linking are also employed.

Next time, additional backup types – Delta or Block-Level Backups, Binary Patch Backups (FastBit), Synthetic Full Backups and Hard Linked Backups

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