Apptainer

This article is a draft

This is not a complete article: This is a draft, a work in progress that is intended to be published into an article, which may or may not be ready for inclusion in the main wiki. It should not necessarily be considered factual or authoritative.

Notices

This page is draft work-in-progress. At the time of writing (August 2022) Apptainer version 1.1 is in release candidate status and the full release may well induce further updates to this page. (This page's current content is correct for Apptainer version 1.0.)

Official Apptainer Documentation

This page is neither exhaustive nor all-features complete and does not replace official documentation, rather, it summarizes basic use, documents some aspects of using Apptainer on Alliance systems, and provides some examples relevant in using Apptainer on Alliance systems. We recommend all users read the official Apptainer documentation concerning the features of Apptainer they are using.

If Currently Using Singularity

We strongly recommend that you start using Apptainer instead of Singularity. SingularityCE (up to v3.9.5) was adopted by The Linux Foundation and renamed to Apptainer with these changes:

• added experimental support for DMTCP checkpointing,
  • NOTE: Support for such is not in Singularity.
• removed support for the --nvccli command line option,
• removed support for apptainer build --remote,
• removed support the SylabsCloud remote endpoint replacing it with a DefaultRemote endpoint with no defined server for library://,
  • NOTE: If the SylabsCloud remote is needed, the previous default can be restored.
• renamed all executable names, paths, etc. having singularity in their names to have apptainer in them,
  • e.g., instead of using the singularity command one uses the apptainer command
  • e.g., the ~/.singularity directory is now ~/.apptainer
• renamed all environment variables having SINGULARITY in their names to have APPTAINER in them,

Should you need to port scripts, etc. to Apptainer, know Apptainer version 1 is backwards compatible with Singularity so switching to Apptainer can be done incrementally.

Using Apptainer

In order to use Apptainer one must first already have a container image, e.g., a .sif file or a "sandbox" directory created previously. If you don't already have a container image, see the section on building an image below.

Loading an Apptainer Module

In order to use the default version of Apptainer available run:

$ module load apptainer

To see the available versions of Apptainer that can be loaded run:

$ module spider apptainer

Running Programs Within a Container

Important Items

sudo

Many users ask about sudo since documentation and web sites often discuss using sudo. Know the ability to use sudo to obtain root permissions is not available on our clusters. Should you require using sudo, consider the following options:

• Install Linux, Apptainer, and sudo in a virtual machine on a system you control so you will be able to have sudo access within such. Build your image(s) on that machine and upload them in order to use them on Alliance systems.
• If appropriate, submit a ticket asking if Alliance staff would be able to help build the image(s), etc. required needing sudo. (Understand that this may or may not be done/possible --but feel free to ask such in a ticket if what you wish to achieve is beyond your means. Additionally, we may respond with other ways to achieve such with may or may not involve Apptainer.)

Important Command Line Options

Software that is run inside a container runs in a different environment, libraries, and tools than what is installed on the host system. It is, therefore, wise to run programs within containers by not importing any environment settings or software defined outside of the container. By default Apptainer will run adopting the shell environment of the host but this can result in issues when running programs inside the container. To work around this when using apptainer run, apptainer shell, apptainer exec, and apptainer instance, consider using one of these options (with more preference to those options listed above other options):

Apptainer Environment Command Line Options

Option	Description
-C	Isolates the running container from all file systems as well as the parent PID, IPC, and environment. Using this option will require using bind mounts if access to filesystems outside of the container is needed.
-c	Isolates the running container from most file systems only using a minimal /dev, an empty /tmp directory, and an empty /home directory. Using this option will require using bind mounts if access to filesystems outside of the container is needed.
-e	Cleans (some) shell environment variables before running container commands and applies settings for increased OCI/Docker compatibility. Using this option also implies the use of these options: --containall, --no-init, --no-umask, --writable-tmpfs.
	When no options are used the environment variables from the parent shell exist as-is inside the container (which can cause issues to occur) and (virtually) all filesystems are also present inside the container.

Another important option one should consider and may need to use Apptainer successfully is the -W or --workdir option. On Alliance clusters and on most Linux systems, /tmp and similar filesystems use RAM --not disk space. Since jobs typically run on our clusters with limited RAM amounts, this can result in jobs getting killed because they consume too much RAM relative to what was requested for the job. A suitable work-around for this is to tell Apptainer to use a real disk space location for its "workdir". This is done by passing the -W option followed by a path to a disk space location where Apptainer can read/write temporary files, etc. For example, suppose one wanted to run a command myprogram in a using an Apptainer container image called myimage.sif with its "workdir" set to /path/to/a/workdir in the filesystem:

$ mkdir -p $HOME/aworkdir
$ apptainer run -C -B /home -W /path/to/a/workdir myimage.sif myprogram

where:

• The workdir directory can be removed if there are no live containers using it.
• When using Apptainer in an salloc, in an sbatch job, or when using [JupyterHub] on our clusters, use ${SLURM_TMPDIR} for the "workdir" location, e.g., -W ${SLURM_TMPDIR}.
  • ASIDE: One should not be running programs (including Apptainer) on a login node: use an interactive salloc job.
• When using bind mounts, see the section on bind mounts below since not all Alliance clusters are all exactly the same concerning the exact bind mounts that are needed to access /home, /project, and /scratch.

Using GPUs

When running software inside a container that requires the use of GPUs it is important to do the following:

• Ensure that you pass the --nv (for NVIDIA hardware) and --rocm (for AMD hardware) to Apptainer commands.
  • These options will ensure the appropriate /dev entries are bind mounted inside the container.
  • These options will locate and bind GPU-related libraries on the host (e.g., so such becomes bind-mounted inside the container) as well as setting the LD_LIBRARY_PATH environment variable to enable the aforementioned libraries will work inside the container.
• Ensure the application using the GPU inside the container was properly compiled to use the GPU and its libraries.
• When needing to use OpenCL inside the container, besides using the aforementioned options use the following bind mount: --bind /etc/OpenCL.

An example of using NVIDIA GPUs within an apptainer container appears later on this page.

Using MPI Programs

If you want to run MPI programs inside a container there are things that need to be done in the host environment in order for such to work. Please see the Running MPI Programs section below for an example of how to run MPI programs inside a container. The official Apptainer documentation has more information concerning how MPI programs can be run inside a container.

Container-Specific Help: apptainer run-help

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Running Software (Preferred): apptainer run

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Interactively Running Software: apptainer shell

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Running Software (Basic): apptainer exec

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Running Daemons: apptainer instance

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Bind Mounts and Persistent Overlays

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Bind Mounts

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Persistent Overlays

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Building an Apptainer Container/Image

Overview

Apptainer "images" can be created in the following formats:

• as an SIF file, or,
• as a "sandbox" directory.

SIF files internally can contain multiple parts where each part is typically a squashfs filesystem (which are read-only and compressed). It is possible for SIF files to contain read-write filesystems and overlay images as well --but such is beyond the scope of this page: see the official Apptainer documentation on how to do such. Unless more advanced methods of building an "image" were used, the Apptainer build command produces a SIF file with a read-only squashfs filesystem when building images. (This is the preferred option since the resulting image remains as-is since it is read-only, and, the image is much smaller since it is compressed. Know that disk reads from that image are done very quickly.)

A "sandbox" directory is a normal directory in the filesystem that starts out as empty and as Apptainer builds the image it adds the files, etc. needed in the image to that directory. The contents of a "sandbox" directory should only be accessed, updated, etc. through the use of Apptainer. One might need to use a "sandbox" directory in situations where one needs to have read-write access to the image itself in order to be able to update the container image. That said, if updates are infrequent, it is typically easier and better to use an SIF and when updates need to be done, build a sandbox image from the SIF file, make the required changes, and then build a new SIF file, e.g.,

$ cd $HOME
$ mkdir mynewimage.dir
$ apptainer build mynewimage.dir myimage.sif
$ apptainer shell --writable mynewimage.dir
Apptainer> # Run commands to update mynewimage.dir here.
Apptainer> exit
$ apptainer build newimage.sif mynewimage.dir
$ rm -rf mynewimage.dir

Using an SIF image is recommended as disk performance (from the container image) will be faster than storing each file, etc. separately on Alliance cluster filesystems (which are set up to handle large files and parallel I/O). Using an SIF file instead of a sandbox image will also only use a quota file count amount of 1 instead of thousands (e.g., images will typically contain thousands of files and directories).

Building a Sandbox Image

In order to build a "sandbox" directory instead of an SIF file instead of providing an SIF file name, instead provide --sandbox DIR_NAME or -s DIR_NAME where DIR_NAME is the name of the to-be-created-directory where you want your "sandbox" image. For example, if the apptainer build command to create an SIF file was:

$ apptainer build bb.sif docker://busybox

then change bb.sif to a directory name, e.g., bb.dir, and prefix such with --sandbox:

$ apptainer build --sandbox bb.dir docker://busybox

Differences between building a "sandbox" image and a (normal) SIF file are:

• the SIF file's image will be contained in a single file, compressed, and read-only,
• the "sandbox" image will be placed in a directory, uncompressed, may contain thousands of files (depending on what exactly is in the image), and will be able to be read-write.

Within an account, using a "sandbox" directory will consume significant amounts of both disk space and file count quotas, thus, if read-write access to the underlying image is not normally required, you are advised to use an SIF instead. Additionally, using an SIF file will have higher disk access speeds to content contained within the SIF file.

Building an SIF Image

NOTE: This section only discusses some basics of creating a simple compressed, read-only SIF file container image. See the Apptainer documentation for more advanced aspects of building images.

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Example Use Cases

Using Docker Images Within an Apptainer Container

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Using Conda Within an Apptainer Container

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Using Spack Within an Apptainer Container

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Using NVIDIA GPUs Within an Apptainer Container

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Running MPI Programs Inside an Apptainer Container

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FAQ

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Message Passing Interface