- Overlayfs allows one, usually read-write, directory tree to be overlaid onto another, read-only directory tree. All modifications go to the upper, writable layer. This type of mechanism is most often used for live CDs but there's a wide variety of other uses.
- The implementation differs from other "union filesystem" implementations in that after a file is opened all operations go directly to the underlying, lower or upper, filesystems. This simplifies the implementation and allows native performance in these cases.
Overlayfs has been in the linux kernel since 3.18.
Overlayfs is enabled in the default kernel and the
overlay module is automatically loaded upon issuing a mount command.
To mount an overlay use the following
# mount -t overlay overlay -o lowerdir=/lower,upperdir=/upper,workdir=/work /merged
The lower directory can actually be a list of directories separated by
:, all changes in the
merged directory are still reflected in
# mount -t overlay overlay -o lowerdir=/lower1:/lower2:/lower3,upperdir=/upper,workdir=/work /merged
To add an overlayfs entry to
/etc/fstab use the following format:
overlay /merged overlay noauto,x-systemd.automount,lowerdir=/lower,upperdir=/upper,workdir=/work 0 0
x-systemd.automount mount options are necessary to prevent systemd from hanging on boot because it failed to mount the overlay. The overlay is now mounted whenever it is first accessed and requests are buffered until it is ready. See Fstab#Automount with systemd.
Sometimes, it is only desired to create a read-only view of the combination of two or more directories. In that case, it can be created in an easier manner, as the directories
work are not required:
# mount -t overlay overlay -o lowerdir=/lower1:/lower2 /merged
upperdir is not specified, the overlay is automatically mounted as read-only.