Terminology

Backup Content

When doing deduplication, there are different strategies to get optimal results in terms of performance and/or deduplication rates. Depending on the type of data, it can be split into fixed or variable sized chunks.

Fixed sized chunking requires minimal CPU power, and is used to backup virtual machine images.

Variable sized chunking needs more CPU power, but is essential to get good deduplication rates for file archives.

The Proxmox Backup Server supports both strategies.

Image Archives: <name>.img

This is used for virtual machine images and other large binary data. Content is split into fixed-sized chunks.

File Archives: <name>.pxar

A file archive stores a full directory tree. Content is stored using the Proxmox File Archive Format (.pxar), split into variable-sized chunks. The format is optimized to achieve good deduplication rates.

Binary Data (BLOBs)

This type is used to store smaller (< 16MB) binary data such as configuration files. Larger files should be stored as image archive.

Caution

Please do not store all files as BLOBs. Instead, use the file archive to store whole directory trees.

Catalog File: catalog.pcat1

The catalog file is an index for file archives. It contains the list of files and is used to speed up search operations.

The Manifest: index.json

The manifest contains the list of all backup files, their sizes and checksums. It is used to verify the consistency of a backup.

Backup Type

The backup server groups backups by type, where type is one of:

vm
This type is used for virtual machines. Typically consists of the virtual machine’s configuration file and an image archive for each disk.
ct
This type is used for containers. Consists of the container’s configuration and a single file archive for the filesystem content.
host
This type is used for backups created from within the backed up machine. Typically this would be a physical host but could also be a virtual machine or container. Such backups may contain file and image archives, there are no restrictions in this regard.

Backup ID

A unique ID. Usually the virtual machine or container ID. host type backups normally use the hostname.

Backup Time

The time when the backup was made.

Backup Group

The tuple <type>/<ID> is called a backup group. Such a group may contain one or more backup snapshots.

Backup Snapshot

The triplet <type>/<ID>/<time> is called a backup snapshot. It uniquely identifies a specific backup within a datastore.

Backup Snapshot Examples
 vm/104/2019-10-09T08:01:06Z
 host/elsa/2019-11-08T09:48:14Z

As you can see, the time format is RFC3399 with Coordinated Universal Time (UTC, identified by the trailing Z).