Glossary

allow_other — the FUSE mount option that lets users other than the one who mounted the pool see it at all. Needs user_allow_other in /etc/fuse.conf. Chapters 7 and 9.

Branch — one real directory, usually a whole drive's root, given to mergerfs as a source. The pool is the branches, viewed together. Chapter 2.

Category — a family of operations sharing one policy: create (new files), search (lookups), action (changes to existing files). Chapter 5.

Control file / control interface — the live configuration of a running pool, read and written through user.mergerfs.* xattrs on the mount point, and readable as text in the hidden .mergerfs pseudo-file at the pool's root. Chapter 8.

ENOSPC — "No space left on device": the error a write gets when its branch is physically full. Chapter 6.

ep (existing path) — the prefix on policies (epmfs, epff, epall) that first restricts candidates to branches where the file's parent directory already exists. Chapter 5.

EXDEV — "Invalid cross-device link": a rename or link tried to cross branches where the policy doesn't allow it. Chapters 5 and 9.

ff (first fit / first found) — the policy that takes the first branch which qualifies: for creation, the first with enough free space; for search, the first holding the path.

FUSE — "Filesystem in Userspace": the kernel mechanism that lets a program like mergerfs provide a filesystem. Needs /dev/fuse. Chapters 2 and 3.

fusermount — the helper that mounts and unmounts FUSE filesystems for unprivileged users: fusermount -u /pool.

minfreespace — the free-space threshold below which create policies stop choosing a branch. Default 4G. A creation threshold, not a reserve. Chapter 6.

moveonenospc — when a write to an existing file fails with ENOSPC, mergerfs moves the file to another branch and retries the write. Chapter 6.

mfs — "most free space": the create policy that picks the branch with the most free space; epmfs is its existing-path form and the default.

Path preservation — the behaviour of ep* policies in renames and links: keep the file's branch consistent with where its path already lives, even if that means moving it between branches. Chapter 5.

Policy — the rule that picks a branch for an operation. Set per category (category.create=mfs) or per function (func.create=ff). Chapter 5.

Pool — the merged mount point: the single directory where all branches appear as one tree. Also, this guide's word for the whole arrangement.

SnapRAID — the scheduled-parity tool most often paired with mergerfs: mergerfs presents; SnapRAID protects. Chapter 10.

Union filesystem — any filesystem that presents several directories as one tree. MergerFS is one; OverlayFS is the kernel's. Chapter 12.

xattr — extended attributes: name/value metadata attached to files. MergerFS uses them both on your files (passed through to the branches) and as its own control interface on the mount point. Chapter 8.