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mergerfs - a featureful union filesystem

mergerfs is a FUSE based union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is similar to mhddfs, unionfs, aufs, DrivePool, etc.

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 for near native IO performance (where supported)
  • Works with filesystems of any size
  • Works with filesystems of almost any type
  • Ignore read-only filesystems when creating files
  • Hard links
  • Hard link copy-on-write / CoW
  • Runtime configurable
  • 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)
  • 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. More specifically by policies.

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.

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 and the docs will be updated.