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Quick Start

Install

First ensure you have the latest version installed.

Configuration

mergerfs has many options and effectively all of them are functional in nature. What that means is that there is no "best" or "fastest" configuration. No "make faster" options. Everything changes behavior. Sometimes those changes in behavior affect performance.

If you don't already know that you have special requirements then use one of the following option sets as it will cover the common use cases.

You use Linux v6.6 or above

  • cache.files=off
  • category.create=pfrd
  • func.getattr=newest
  • dropcacheonclose=false

See Known Issues and Bugs#Software Using mmap for more details why the Linux and mergerfs version matter and its relation to mmap.

NOTE: This "auto mmap" feature is only supported in mergerfs v2.41.0 and above. For older versions see below or the docs from that release.

You use Linux v6.5 or below

You need mmap

To keep the details in a single location: see Known Issues and Bugs#Software Using mmap for more details.

  • cache.files=auto-full
  • category.create=pfrd
  • func.getattr=newest
  • dropcacheonclose=true

You don't need mmap

  • cache.files=off
  • category.create=pfrd
  • func.getattr=newest
  • dropcacheonclose=false

Usage

For some suggestions on branch setup see the page on branches.

Web UI

mergerfs-webui provides a browser-based alternative for creating and managing mounts on Linux. It can edit startup configuration and inspect and change settings on running mounts. See the mergerfs-webui documentation for installation and secure access guidance. Runtime changes alone do not persist across remounts.

Command Line

# Two positional options: `:` colon separated branches and mountpoint
mergerfs -o opt[,opt...] /mnt/hdd/disk0:/mnt/hdd/disk1:/mnt/hdd/disk2 /media
# or list of positional options where the last is the mountpoint
mergerfs -o opt[,opt...] /mnt/hdd/disk0 /mnt/hdd/disk1 /mnt/hdd/disk2 /media
# or set mountpoint via -o mountpoint
mergerfs -o mountpoint=/media /mnt/hdd/disk0 /mnt/hdd/disk1 /mnt/hdd/disk2

mergerfs -o cache.files=off,category.create=pfrd,func.getattr=newest,dropcacheonclose=false /mnt/hdd/disk0:/mnt/hdd/disk1:/mnt/hdd/disk2 /media

/etc/fstab

/mnt/hdd/disk0:/mnt/hdd/disk1:/mnt/hdd/disk2 /media mergerfs cache.files=off,category.create=pfrd,func.getattr=newest,dropcacheonclose=false 0 0

systemd mount dependencies

On a systemd based system, no custom service or setup script is needed when the branch filesystems and mergerfs are all configured in /etc/fstab. Add x-systemd.requires-mounts-for= once for each branch mount target in the mergerfs entry:

LABEL=disk0 /mnt/hdd/disk0 ext4 defaults 0 2
LABEL=disk1 /mnt/hdd/disk1 ext4 defaults 0 2
LABEL=disk2 /mnt/hdd/disk2 ext4 defaults 0 2
/mnt/hdd/disk0:/mnt/hdd/disk1:/mnt/hdd/disk2 /media mergerfs cache.files=off,category.create=pfrd,func.getattr=newest,dropcacheonclose=false,x-systemd.requires-mounts-for=/mnt/hdd/disk0,x-systemd.requires-mounts-for=/mnt/hdd/disk1,x-systemd.requires-mounts-for=/mnt/hdd/disk2 0 0

Replace the example labels and filesystem types with those used by the system. systemd creates Requires= and After= dependencies from the mergerfs mount to mnt-hdd-disk0.mount, mnt-hdd-disk1.mount, and mnt-hdd-disk2.mount. It starts those mounts first and does not start the mergerfs mount if a required mount fails. This checks the mount units, not merely whether the /mnt/hdd/diskN directories exist.

After editing /etc/fstab, reload the generated units:

sudo systemctl daemon-reload

The dependencies apply the next time /media is mounted and on subsequent boots.

When using a service such as the mergerfs-media.service example below instead of an /etc/fstab mergerfs entry, add the equivalent directive to its [Unit] section:

RequiresMountsFor=/mnt/hdd/disk0 /mnt/hdd/disk1 /mnt/hdd/disk2

RequiresMountsFor= adds both the requirement and ordering dependencies, so separate Requires= and After= entries for these mounts are unnecessary.

/etc/fstab w/ config file

For more complex setups it can be useful to separate out the config.

/etc/fstab

/etc/mergerfs/branches/media/* /media mergerfs config=/etc/mergerfs/config/media.ini

/etc/mergerfs/config/media.ini

media.ini
cache.files=off
category.create=pfrd
func.getattr=newest
dropcacheonclose=false
# comments and empty lines are supported

/etc/mergerfs/branches/media/

Create a bunch of symlinks to point to the branch. mergerfs will resolve the symlinks and use the real path.

ls -lh /etc/mergerfs/branches/media/*

lrwxrwxrwx 1 root root 14 Aug  4  2023 disk0 -> /mnt/hdd/disk0
lrwxrwxrwx 1 root root 14 Aug  4  2023 disk1 -> /mnt/hdd/disk1
lrwxrwxrwx 1 root root 14 Aug  4  2023 disk2 -> /mnt/hdd/disk2

systemd (simple)

/etc/systemd/system/mergerfs-media.service

mergerfs-media.service
[Unit]
Description=mergerfs /media service
RequiresMountsFor=/mnt/hdd/disk0 /mnt/hdd/disk1 /mnt/hdd/disk2
After=local-fs.target network.target

[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
  -f \
  -o cache.files=off \
  -o category.create=pfrd \
  -o func.getattr=newest \
  -o dropcacheonclose=false \
  /mnt/hdd/disk0 \
  /mnt/hdd/disk1 \
  /mnt/hdd/disk2 \
  /media
ExecStop=/usr/bin/umount /media
# Or if you need fusermount
# ExecStop=/usr/bin/mergerfs-fusermount -u /media
Restart=on-failure

[Install]
WantedBy=default.target

systemd (w/ setup script)

Since it isn't well documented otherwise: if you wish to do some setup before you mount mergerfs follow this example.

setup-for-mergerfs

/usr/local/bin/setup-for-mergerfs

setup-for-mergerfs
#!/usr/bin/env sh

# Perform setup
/bin/sleep 10

# Report back to systemd that things are ready
/bin/systemd-notify --ready

setup-for-mergerfs.service

/etc/systemd/system/setup-for-mergerfs.service

setup-for-mergerfs.service
[Unit]
Description=mergerfs setup service

[Service]
Type=notify
RemainAfterExit=yes
ExecStart=/usr/local/bin/setup-for-mergerfs

[Install]
WantedBy=default.target

mergerfs-media.service

/etc/systemd/system/mergerfs-media.service

mergerfs-media.service
[Unit]
Description=mergerfs /media service
RequiresMountsFor=/mnt/hdd/disk0 /mnt/hdd/disk1 /mnt/hdd/disk2
Requires=setup-for-mergerfs.service
After=local-fs.target network.target setup-for-mergerfs.service

[Service]
Type=simple
KillMode=none
ExecStart=/usr/bin/mergerfs \
  -f \
  -o cache.files=off \
  -o category.create=pfrd \
  -o func.getattr=newest \
  -o dropcacheonclose=false \
  /mnt/hdd/disk0 \
  /mnt/hdd/disk1 \
  /mnt/hdd/disk2 \
  /media
ExecStop=/usr/bin/umount /media
# Or if you need fusermount
# ExecStop=/usr/bin/mergerfs-fusermount -u /media
Restart=on-failure

[Install]
WantedBy=default.target