Traditional backup tools can mostly be subdivided by the following characteristics:

• file-based vs. image-based
  Image-based solutions make sure everything is backed up, but are potentially difficult to restore on other (less powerful) hardware. Additionally, creating images by using traditional tools like dd requires the disk that is being backed up to be unmounted (to avoid consistency issues). This makes image-based backups better suited for filesystems that allow doing advanced operations like snapshots or zfs send-style images that contain a consistent snapshot of the data of interest. For file-based tools there is also a distinction between tools that exactly replicate the source file structure in the backup target (e.g. rsync or rdiff-backup) and tools that use an archive format to store backup contents (tar).

• networked vs. single-host
  Networked solutions allow backing up multiple hosts and to some extent allow for centralized administration. Traditionally, a dedicated client is required to be installed on all machines to be backed up. Networked solutions can act pull-based (server gets backups from the clients) or push-based (client sends backup to server). Single-Host solutions consist of a single tool that is being invoked to backup data from the current host to a target storage. As this target storage can be a network target, the distinction between networked and single-host solutions is not exactly clear.

• incremental vs. full
  Traditionally, tools either do an actual 1:1 copy (full backup) or copy “just the differences“ which can mean anything from “copy all changed files” to “copy changes from within files”. Incremental schemes allow multiple backup states to be kept without needing much disk space. However, traditional tools require that another full backup be made in order to free space used by previous changes.

Modern tools mostly advance things on the incremental vs. full front by acting incremental forever without the negative impacts that such a scheme has when realized with traditional tools. Additionally, modern tools mostly rely on their own/custom archival format. While this may seem like a step back from tools that replicate the file structure, there are numerous potential advantages to be taken from this:

• Enclosing files in archives allows them and their metadata to be encrypted and portable across file systems.

• Given that many backups will eventually be stored to online storages like Dropbox, Mega, Microsoft One Drive or Google Drive, the portability across file systems is especially useful. Even when not storing backups online, portability ensures that backup data can be copied by easy operations like cp without damaging the contained metadata. Given that online stores are often not exactly trustworthy, encryption is also required.

Abstract

This article attempts to compare three modern backup tools with respect to their features and performance. The tools of interest are Borg, Bupstash and Kopia.