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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, and aufs.

Features

  • Logicially combine numerous filesystems/paths into a single mount point
  • 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
  • Works with filesystems of any size
  • Works with filesystems of almost any type
  • Ignore read-only filesystems when creating files
  • 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 readonly filesystem like OverlayFS
  • File whiteout
  • RAID like parity calculation
  • Redundency
  • Splitting of files across branches

How it works

mergerfs logically merges multiple filesystem paths together. 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 chant attributes or remove the file the behavior may be applied to all instances found.

Read more about policies here.

Visualization

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