# mergerfs - a featureful union filesystem

**mergerfs** is a
[FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) based
[union filesystem](https://en.wikipedia.org/wiki/Union_mount) geared
towards simplifying storage and management of files across numerous
commodity storage devices. It is similar to [**mhddfs**, **unionfs**,
**aufs**, **DrivePool**, etc.](project_comparisons.md)


## Features

* Logically combine numerous filesystems/paths into a single
  mount point (JBOFS: Just a Bunch of FileSystems)
* Combine paths of the same or different filesystems
* Ability to add or remove filesystems/paths without impacting the
  rest of the data
* Unaffected by individual filesystem failure
* Configurable file selection and creation placement
* File IO [passthrough](config/passthrough.md) for near native IO
  performance (where supported)
* Works with filesystems of any size
* Works with filesystems of [almost any
  type](faq/compatibility_and_integration.md#what-filesystems-can-be-used-as-branches)
* Ignore read-only filesystems when creating files
* [Hard links](faq/technical_behavior_and_limitations.md#do-hard-links-work)
* Hard link [copy-on-write / CoW](config/link_cow.md)
* [Runtime configurable](runtime_interface.md)
* Support for extended attributes (xattrs)
* Support for file attributes (chattr)
* Support for POSIX ACLs


## Non-features

* Read/write overlay on top of read-only filesystem like OverlayFS
* File whiteout
* RAID like parity calculation (see [SnapRAID](https://www.snapraid.it))
* Redundancy
* Splitting of files across branches


## How it works

mergerfs logically merges multiple filesystem paths together. Not
block devices, not filesystem mounts, just paths. It acts as a proxy
to the underlying filesystem paths. Combining the behaviors of some
functions and being a selector for others.

When the contents of a directory are requested mergerfs combines the
list of files from each directory, deduplicating entries, and returns
that list.

When a file or directory is created a policy is first run to determine
which branch will be selected for the creation.

For functions which change attributes or remove the file the behavior
may be applied to all instances found.

The way in which mergerfs behaves is controlled by the
[config/options/settings](config/options.md). More specifically by
[policies](config/functions_categories_policies.md).


### Visualization

```
A         +      B        =       C
/disk1           /disk2           /merged
|                |                |
+-- /dir1        +-- /dir1        +-- /dir1
|   |            |   |            |   |
|   +-- file1    |   |            |   +-- file1
|                |   +-- file2    |   +-- file2
|                |   +-- file3    |   +-- file3
|                |                |
+-- /dir2        |                +-- /dir2
|   |            |                |   |
|   *-- file4    |                |   +-- file4
|                |                |
|                +-- /dir3        +-- /dir3
|                |   |            |   |
|                |   +-- file5    |   +-- file5
|                |                |
+-- file6        |                +-- file6
+-- file7        +-- file7        +-- file7
```


## Getting Started

Head to the [quick start guide](quickstart.md).


## About This Documentation

* Like the software the documentation changes over time. Ensure that
  you are reading the documentation related to the version of the
  software you are using.
* The documentation is explicit, literal, and reasonably thorough. If
  a suspected feature is not mentioned it does not exist. Do not read
  into the wording. What is described is how it functions. If you feel
  like something is not explained sufficiently or missing please [ask
  in one of the supported forums](support.md#contact-issue-submission)
  and the docs will be updated.