apptainer-overlay

Create and manage writable overlay filesystems for containers.

TLDR

Add a writable overlay to an existing SIF image

$ apptainer overlay create -s [size] [path/to/image.sif]

Create a standalone EXT3 writable overlay image

$ apptainer overlay create -s [size] [path/to/overlay.img]

Create a sparse overlay image

$ apptainer overlay create -s [size] -S [path/to/overlay.img]

Create an overlay for use with fakeroot

$ apptainer overlay create -f -s [size] [path/to/overlay.img]

Create an overlay with a specific directory in the layout

$ apptainer overlay create --create-dir [path/to/directory] [path/to/overlay.img]

SYNOPSIS

apptainer overlay create [options] image

apptainer overlay manages EXT3 writable overlay filesystems for Apptainer containers. Overlays allow persistent writes on top of immutable SIF container images without modifying the original image.
Overlays can be embedded directly into a SIF file or created as standalone image files that are attached at runtime. This enables persistent storage of modifications, logs, and user data while maintaining container immutability.

PARAMETERS

-s, --size size

Size of the overlay image in MiB

-S, --sparse

Create a sparse file instead of allocating full disk space

-f, --fakeroot

Create overlay for use with fakeroot

--create-dir path

Create a specific directory in the overlay layout

CAVEATS

Overlays require filesystem support for loop devices and EXT3. Sparse overlays save disk space but may cause performance issues with some workloads. Overlay size cannot be easily changed after creation.