a featureful union filesystem
mergerfs is a union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is similar to mhddfs, unionfs, and aufs.
Configurable behaviors / file placement
Ability to add or remove filesystems at will
Resistance to individual filesystem failure
Support for extended attributes (xattrs)
Support for file attributes (chattr)
Runtime configurable (via xattrs)
Works with heterogeneous filesystem types
Moving of file when filesystem runs out of space while writing
Ignore read-only filesystems when creating files
Turn read-only files into symlinks to underlying file
Hard link copy-on-write / CoW
Support for POSIX ACLs
Misc other things
HOW IT WORKS
mergerfs logically merges multiple paths together. Think a union of sets. The file/s or directory/s acted on or presented through mergerfs are based on the policy chosen for that particular action. Read more about policies below.
mergerfs does NOT support the copy-on-write (CoW) or whiteout behaviors found in aufs and overlayfs. You can not mount a read-only filesystem and write to it. However, mergerfs will ignore read-only drives when creating new files so you can mix read-write and read-only drives. It also does NOT split data across drives. It is not RAID0 / striping. It is simply a union of other filesystems.
branch: A base path used in the pool.
pool: The mergerfs mount. The union of the branches.
relative path: The path in the pool relative to the branch and mount.
function: A filesystem call (open, unlink, create, getattr, rmdir, etc.)
category: A collection of functions based on basic behavior (action, create, search).
policy: The algorithm used to select a file when performing a function.
path preservation: Aspect of some policies which includes checking the path for which a file would be created.
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