> For the complete documentation index, see [llms.txt](https://docs.duplicati.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.duplicati.com/technical-details/understanding-backup.md).

# Understanding Backup

When running a backup, the goal is to create a new "snapshot" of the filesystem. In some cases the operating system is capable of providing a consistent disk-level snapshot, but if not, the goal is to get as close to a snapshot as possible. Because it is not guaranteed that a true disk-level snapshot was made, Duplicati refers to these "snapshots" as versions.

Each backup will produce a "filelist" (aka `.dlist`) file on the remote storage as well as one or more block volumes (aka `.dblock`) and usually also one or more index volumes (aka `.dindex`). The filelists explain how to recreate the files that are in this particular version, and the block volumes contains the actual file contents, split into blocks.

The index volumes are optional, and not required for restores, but are created by default as they reduce the amount of data that needs to be processed if the [local database](/database-and-storage/the-local-database.md) is lost.

The backup process is described in the next section and explains how these files are constructed.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.duplicati.com/technical-details/understanding-backup.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.