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Building a Bootc-Inspired Immutable Linux System for Debian/Ubuntu: A Comprehensive Implementation PlanExecutive SummaryThe landscape of Linux system management is evolving, moving towards immutable operating system architectures that promise enhanced stability, security, and simplified operations. bootc exemplifies this trend by leveraging OCI container images for the delivery and management of entire host systems, including the kernel and bootloader. This report outlines a comprehensive plan to achieve a feature-complete bootc-like alternative for Debian and Ubuntu distributions. The approach involves orchestrating native Debian/Ubuntu tools such as debootstrap, live-build, OverlayFS, and Btrfs with snapper to mimic bootc's core functionalities: immutable root filesystems, atomic updates, robust rollback capabilities, and container-native workflows. This blueprint aims to provide technical professionals with the necessary guidance to implement a highly resilient and manageable Linux environment, emphasizing reproducibility and consistency across deployments.1. Introduction to Immutable Linux Systems and Bootable ContainersThe traditional model of Linux system administration, characterized by package-by-package updates and mutable root filesystems, presents inherent challenges in maintaining consistency, ensuring security, and scaling deployments. Configuration drift, broken dependencies, and complex rollback procedures are common pain points. The emergence of immutable operating systems and bootable containers offers a transformative approach to address these issues.1.1 The Vision: Unified DevOps and Simplified ManagementBootable containers represent a significant evolution of container technologies, extending the successful application container model to encompass the entire operating system, including the Linux kernel, bootloader, and drivers.1 This paradigm shift aims to unify application and OS management, providing a streamlined DevOps approach where the entire system can be managed through familiar container-based tooling, GitOps, and continuous integration/continuous delivery (CI/CD) pipelines.1 This unified strategy helps in addressing the complexities of managing Linux at scale, facilitating the consistent deployment of patches and bridging the operational gaps between development and operations teams.1A fundamental aspect of this vision is the shift from package-centric to image-centric OS management. Unlike traditional Debian/Ubuntu distributions that rely on apt and dpkg for individual package updates, bootc explicitly utilizes "standard OCI/Docker containers as a transport and delivery format for base operating system updates".1 This signifies a philosophical move towards treating the operating system as a singular, atomic, and versioned image. Managing the OS as a cohesive image simplifies the overall system state, significantly reducing the complexity associated with dependency management and mitigating configuration drift. This approach leads to more predictable and consistent system deployments, which is particularly beneficial in large-scale or automated environments where system consistency is paramount.Furthermore, security is substantially simplified. By shipping the OS in the form of container images, advancements in container security tools can be leveraged to perform comprehensive patching, scanning, validation, and signing across the entire stack, including the kernel, drivers, and bootloader.1 The inherent immutability of the OS, once deployed with a read-only root filesystem, further enhances security by minimizing the attack surface and preventing unauthorized or accidental modifications. This directly improves system integrity and compliance by ensuring that the deployed system remains in a known, verified state. The concept of "build once—deploy everywhere" is also greatly enhanced, allowing the same tooling and processes to be used for creating application containers, virtual machine images, or bare metal installers.21.2 Understanding Immutability: Read-Only Root and Atomic UpdatesAt the core of bootable containers lies the principle of immutability, characterized by "transactional, in-place operating system updates using OCI/Docker container images".1 This means that once a system is deployed, most of its directories, such as /usr, are mounted as read-only. This read-only nature provides a stable and predictable environment, preventing runtime modifications that could lead to system instability or security vulnerabilities.1 During the initial image build process, however, the entire filesystem is writable, allowing for the necessary configuration and installation of components.1Atomic updates are a cornerstone of immutable systems. This mechanism ensures that an update either completes entirely and successfully, or the system remains in its prior, consistent state, preventing partial or corrupted installations.7 This is akin to a "database transaction," where changes are prepared in an isolated environment before being committed as a single, indivisible unit. This "all or nothing" approach guarantees system consistency, preventing scenarios where a partial update could render a system unbootable. This transactional model significantly enhances system reliability and minimizes downtime, which is critical for production environments and large-scale deployments where system consistency is paramount. Typically, this is achieved by building a new, complete system image, deploying it alongside the existing one, and then performing a single, atomic switch—often via a system reboot—to activate the new image.7A significant advantage of this atomic update model is simplified rollback. If a newly applied update introduces issues, fails to boot, or exhibits unexpected behavior, the system can easily and quickly revert to the previous, known-good operating system image.5 This capability provides a crucial safety net, minimizing downtime and recovery efforts.Despite the emphasis on immutability, certain directories must remain writable for system functionality and user interaction. These include /etc (for configuration files) and /var (for variable data such as logs, temporary files, and user home directories).1bootc addresses the challenge of /etc by employing a sophisticated 3-way merge during upgrades to intelligently preserve local modifications while integrating new upstream configurations.6 For /var, content included in the container image is treated like a Docker VOLUME, meaning it is unpacked only during the initial deployment and is not automatically updated with subsequent image changes.6 This highlights a fundamental tension between the desire for a fully immutable system and the practical necessity of mutable state, a key design challenge for any alternative implementation.1.3 bootc as a Reference: Architecture and ostree Foundationbootc serves as a command-line interface (CLI) tool, complemented by systemd services and timers, designed to manage bootable containers.1 Its core function revolves around downloading and queuing operating system updates. The underlying technology that provides bootc's stable and transactional update capabilities is the ostree project, a mature system that has powered robust OS updates for many years.3ostree operates by managing multiple bootable, versioned filesystem trees, which are parallel-installed into a dedicated /ostree directory.11 Conceptually, ostree functions much like Git, but for OS binaries, providing content-addressed storage for the entire operating system tree.14 This content-addressed storage allows for highly efficient storage and atomic updates by only storing changes once. This is a fundamental difference from traditional package managers or even simple filesystem snapshots. While tools like snapper with Btrfs provide filesystem-level snapshots, ostree operates at a higher, content-addressed tree level, with built-in HTTP replication and static deltas for efficient distribution of OS updates.15 This makes ostree uniquely suited for managing numerous versions of a base operating system efficiently.ostree-based systems enforce a read-only paradigm; any changes to the system, whether an upgrade, downgrade, or software installation, involve checking out a new, complete filesystem tree. During this process, a 3-way merge is performed for configuration files in /etc to reconcile local modifications with upstream changes.11 The newly checked-out tree then becomes active upon a system reboot. The bootc install command is crucial in bridging the gap between a standard OCI container image and a bootable system. It executes specific tooling logic embedded within the container image to dynamically set up the filesystem and bootloader, typically requiring --privileged access.16bootc further integrates with the bootupd project to abstract and manage bootloader installations and upgrades, ensuring a consistent boot environment.16A significant strength of bootc lies in its seamless integration with the existing OCI/Docker container ecosystem.1 This means that the entire OS image is treated as a standard container artifact, enabling it to be built, signed, distributed, and scanned using familiar container tooling and registries. This approach unifies application and OS deployment workflows, simplifying CI/CD and GitOps practices across the entire system stack, from applications down to the kernel.1.4 Why a Debian/Ubuntu Alternative? Goals and ScopeThe explicit requirement for this report is to develop a bootc-like alternative tailored specifically for Ubuntu or Debian distributions. This necessitates leveraging the existing, well-established tools and methodologies within these ecosystems rather than attempting to directly port bootc's ostree foundation.Ubuntu Base provides a minimal root filesystem, designed for creating custom images. It offers a functional user-space environment with full support for installing additional software via apt-get.17 Similarly, Debian's debootstrap is a foundational utility for creating a basic Debian system within a chroot environment, capable of cross-architecture bootstrapping.18 The Debian Live project's live-build is a comprehensive tool suite specifically designed for constructing various Debian Live system images, including ISOs and USB stick images.20The strategy involves leveraging this existing ecosystem rather than attempting to reinvent the wheel. The goal is to achieve bootc-like functionality by composing these well-understood Debian/Ubuntu tools. This approach prioritizes integration with the native ecosystem over a completely new, opinionated stack. This means identifying the most suitable tools for each bootc feature – for instance, debootstrap or ubuntu-base for creating the base filesystem, live-build for image assembly, OverlayFS for enforcing immutability, and Btrfs with snapper for transactional updates and robust rollbacks. The feasibility and maintainability of this alternative depend heavily on the maturity and flexibility of these existing tools and how effectively they can be orchestrated to provide a cohesive, automated workflow comparable to bootc.To provide a clear understanding of the architectural and tooling differences, the following table compares bootc's established approach with the proposed Debian/Ubuntu alternative:Table 1: bootc vs. Debian/Ubuntu Alternative: Feature ComparisonFeaturebootc (Reference)Debian/Ubuntu Alternative (Proposed)Immutability MechanismOSTree (content-addressed, read-only /usr)SquashFS (read-only base) + OverlayFS (writable overlay)Update StrategyAtomic (new rootfs deployment)Atomic (A/B partitioning or Btrfs snapshots)Configuration Management (/etc)3-way merge with /usr/etcOverlayFS copy-up (manual merge/scripting or snapper diff/rollback)Variable Data (/var)Persists by default (like Docker VOLUME)Dedicated partition/Btrfs subvolumeRollback MechanismOSTree deployments (bootloader switch)A/B bootloader switch or Btrfs snapshot bootImage FormatOCI Container ImageCustom (e.g., QCOW2, ISO) then OCI conversionBuild Toolingbootc CLIdebootstrap/live-build/custom scriptsUpdate Triggersystemd timers/manualSystemd timers/manual/custom hooksThis comparison highlights where direct functional parallels can be drawn and where different, native Debian/Ubuntu approaches are necessary. It underscores the complexity and design choices inherent in building a "feature complete" alternative.2. Core Components: Building the Immutable Base ImageCreating an immutable Linux system for Debian or Ubuntu begins with establishing a minimal, reproducible base image. This involves selecting appropriate tools for root filesystem generation, integrating a custom kernel and initial RAM filesystem (initramfs), and implementing the read-only root using OverlayFS.2.1 Minimal Root Filesystem CreationFor Debian-based systems, debootstrap is the standard and foundational tool for creating a minimal Debian system within a specified directory.18 It operates by downloading packages from a Debian repository and unpacking them to build a basic environment suitable for a chroot. debootstrap is versatile, supporting the creation of systems for different architectures, making it suitable for cross-architecture image building.18For Ubuntu, Ubuntu Base provides pre-built minimal rootfs tarballs. These are designed to be extracted onto ext2, ext3, or ext4 partitions.17Ubuntu Base delivers a functional user-space environment with full support for installing additional software via apt-get. However, to keep its footprint as small as possible, it ships without local users, a default syslog server, or a complete Python environment.17An example of a minimalist Debian-based image optimized for containers is minideb. This image achieves a significantly smaller size by judiciously removing packages not typically required in containerized environments, such as hardware-related tools or full init systems, and by pruning unnecessary files like documentation and man pages.22minideb also includes a convenient install_packages command that streamlines apt operations by handling prompts, cleaning up metadata, and retrying downloads, which is particularly useful in automated build pipelines.22The choice of a minimal base image involves a trade-off between size and functionality. While minideb and Ubuntu Base offer substantially reduced footprints 22, this size reduction often comes at the cost of removing components that are typically considered essential for a fully bootable operating system (e.g., systemd, OpenSSH, or full user management tools).17 Consequently, the image builder must carefully select and explicitly re-include these packages during the build process to achieve a "feature complete" bootable system, which adds a layer of complexity to the overall image creation. This aggressive size optimization necessitates the explicit re-inclusion of core OS components, which can increase build complexity and introduce potential for missing dependencies if not meticulously managed.These tools (debootstrap or Ubuntu Base) serve as the "first layer" of immutability. They provide a clean, reproducible base filesystem, which is a pristine starting point before applying immutability layers like SquashFS and OverlayFS. Any subsequent modifications or additions to this base must be performed declaratively and consistently to ensure the reproducibility and integrity of the final immutable image. The quality and reliability of the final bootable container are directly tied to this initial rootfs creation process. Automating this step, perhaps within live-build configurations or custom scripts, is vital for achieving a bootc-like automated workflow.2.2 Custom Kernel and Initramfs Integrationbootc images inherently include their own Linux kernel, which is crucial for their ability to be deployed across diverse environments, from bare metal to virtual machines.2 This self-contained nature of the OS image, including its kernel, is a key characteristic to replicate.The initramfs (initial RAM filesystem) plays a pivotal role in the Linux boot process. It is a small filesystem that the kernel loads into RAM very early, before the actual root filesystem is mounted.24 The initramfs contains an initialization program (typically /init) responsible for critical early boot tasks, such as locating and mounting the real root filesystem, loading necessary kernel modules (especially those required for specific filesystems like OverlayFS or Btrfs, or for hardware drivers), and handling scenarios like encrypted root partitions.24 The initramfs can either be bundled directly into the kernel image, which increases kernel size but simplifies the bootloader's role, or loaded externally by the bootloader as a separate archive.24For Debian and Ubuntu, well-documented procedures exist for building custom kernels. This typically involves obtaining the kernel source code, configuring it (often using make menuconfig for interactive selection or make defconfig for a default configuration), and then packaging it into .deb files (make bindeb-pkg for Debian or debian/rules binary for Ubuntu).27 Essential kernel configuration options, such as CONFIG_BLK_DEV_INITRD, must be enabled to ensure initramfs support.24 The live-build tool suite, commonly used for creating live images, supports the inclusion of these custom-built kernels in the generated images.31The initramfs acts as the orchestrator for immutability. It must contain the logic and necessary kernel modules (e.g., for OverlayFS and SquashFS) to mount the read-only base layer and the writable overlay before the main init process (like systemd) takes control.33 This makes the initramfs a critical component for establishing the read-only root functionality early in the boot process, before userspace fully initializes.Kernel customization further allows for a minimal footprint and specific features. While bootc ships its own kernel 2, building a custom kernel for Debian/Ubuntu provides granular control over the final image. This enables tailoring the kernel to specific hardware by building necessary drivers directly into the kernel or as modules, ensuring that required filesystem features (SquashFS, OverlayFS, Btrfs) are compiled in as built-in components to simplify initramfs requirements, and potentially stripping out unnecessary drivers to reduce the overall image size. This aligns with the minimalist approach seen in minideb and contributes to an optimized OS environment.22 This level of kernel customization is characteristic of appliance-like or embedded systems, aligning with the bootc philosophy of a tightly controlled and optimized OS environment.2.3 Implementing Read-Only Root with OverlayFSA foundational characteristic of immutable systems is their read-only core.7bootc enforces this by mounting most directories as read-only after deployment, with /etc and /var as the primary exceptions for mutable state.1 To replicate this behavior in Debian/Ubuntu, OverlayFS is a key technology.OverlayFS is a union filesystem that effectively merges multiple directories into a single, unified view.37 It operates with a lowerdir, which represents the read-only base filesystem, and an upperdir, a read-write layer where all modifications and new files are stored.37 When a file exists in both layers, the version from the upperdir takes precedence.39For the read-only base, SquashFS is an ideal choice. It is a highly compressed, read-only filesystem format widely used in live systems and embedded devices due to its efficiency and integrity.33 A SquashFS image can serve as the lowerdir for OverlayFS, providing the immutable core of the operating system.The overlayroot package in Ubuntu, and similar custom scripts like immutablefs for Debian/Raspberry Pi, simplify the configuration of a read-only root filesystem using OverlayFS.38 By default, overlayroot can be configured to use tmpfs (a RAM-based filesystem) as the upperdir. In this configuration, any changes made to the system during runtime are ephemeral and will be lost upon reboot.38 This behavior is desirable for specific use cases, such as kiosks or disposable testing environments. overlayroot can also be temporarily disabled by passing overlayroot=disabled as a kernel boot argument, allowing for temporary write access when necessary (e.g., for system updates).38 Debian's overlay-boot script also provides functionality for creating "subhosts" with an overlay root filesystem, offering administrative sandboxing.44OverlayFS serves as the core immutability enforcer. The combination of a SquashFS base image and an OverlayFS mount is the primary technical mechanism for achieving a read-only root filesystem while still providing a functional, writable environment. This setup ensures that the core operating system remains untouched, while temporary changes (such as package installations, configuration modifications, or runtime data) are written to the upperdir.39 This guarantees the immutability of the underlying base OS.However, the ephemeral nature of OverlayFS writes, when the upperdir is backed by tmpfs, requires careful consideration. While advantageous for certain scenarios, a "feature complete" bootc alternative necessitates persistence for critical directories like /etc and /var. This means that for a persistent immutable system, the upperdir must be configured to reside on a persistent storage device (e.g., a dedicated disk partition) rather than tmpfs.36 This design choice directly influences the persistence strategy for mutable system state and requires careful planning of filesystem layout and boot-time mounting procedures to ensure data longevity where needed.3. Managing Persistent State in an Immutable EnvironmentA critical challenge in designing an immutable operating system is effectively managing mutable system state, particularly in directories like /etc and /var, which must remain writable for system functionality and user data persistence. bootc addresses this through specific mechanisms, and a Debian/Ubuntu alternative must devise comparable strategies.3.1 Strategies for /etc PersistenceIn bootc systems, /etc is a mutable and persistent directory. The OSTree project, which underpins bootc, handles upgrades by performing a sophisticated 3-way merge for /etc. This process ensures that locally modified configuration files are intelligently retained while new default files from the updated image are applied, minimizing conflicts.6For traditional Debian/Ubuntu systems, /etc is inherently writable.45 Many system components and applications modify files within /etc at runtime, such as adjtime, blkid.tab, LVM configuration, passwd and shadow for user management, and resolv.conf.45 When implementing a read-only root filesystem, these files require special handling. Common strategies include symlinking these mutable files to a writable location within /var or configuring applications to use alternative paths for their dynamic configuration data.45 For instance, apt-get can be configured with DPkg hooks in /etc/apt/apt.conf to automatically remount the root filesystem as read/write before package operations and then revert it to read-only afterwards, enabling seamless updates on an otherwise immutable base.45The complexity of /etc merging without ostree's 3-way merge is a notable challenge. OSTree's 3-way merge for /etc 6 is an advanced feature that automatically reconciles local changes with upstream updates. In contrast, a standard OverlayFS, when used for /etc, typically employs a "copy-up" mechanism.39 Any local modification to a file in the lowerdir results in a new copy being created in the upperdir, effectively masking the original. This simpler approach lacks the intelligent merging capabilities of ostree for handling conflicts between local modifications and upstream changes. Manual workarounds like symlinking or apt hooks 45 are less elegant and scalable. Replicating bootc's robust /etc management without adopting ostree would likely require significant custom scripting or integration with external configuration management tools (e.g., Ansible, Puppet) that apply configurations declaratively after each update, adding considerable complexity to the overall solution. This represents a functional gap that needs careful consideration in the design.If Btrfs is chosen as the underlying filesystem for persistence, snapper can play a significant role in /etc management. snapper can be configured to take pre- and post-snapshots around system changes, including modifications to /etc (if /etc is configured as a Btrfs subvolume).8 While snapper does not perform an automatic 3-way merge like ostree, it enables the comparison of differences (snapper diff) between snapshots 46 and allows for rolling back /etc changes to a previous state.8 This provides a safety net and a mechanism for recovery, though true conflict resolution for complex merges may still require manual intervention. The integration of Btrfs subvolumes for /etc with snapper provides snapshotting of configuration changes, which allows for diffing and rolling back /etc, but still requires manual conflict resolution for true merges.3.2 Handling /var and /home for Variable DataBeyond /etc, directories like /var (for logs, caches, and temporary data) and /home (for user data) are inherently mutable and must persist across operating system updates. In bootc systems, /var and /home (which may be internally mapped to /var/home) are designed to be mutable and persist across OS updates.1 It is important to note that content included in /var within the container image is treated like a Docker VOLUME, meaning its contents are unpacked only from the initial image and are not automatically updated with subsequent image changes.6For a read-only root filesystem, directories such as /var, /home, /srv, and /tmp must be writable.45 The recommended approach for ensuring their persistence is to mount them as separate filesystems or, ideally, as dedicated Btrfs subvolumes.45 Btrfs subvolumes are flexible, directory-like structures that can be mounted independently and snapshotted separately.48 This isolation ensures that critical mutable data—including user home directories, system logs, caches, databases, and even applications installed in /opt or /usr/local—persists across updates or rollbacks of the root filesystem.49 For specific use cases, such as embedded systems or security-hardened environments, /var/log could even be mounted as a tmpfs to make log files non-persistent, ensuring a clean slate after each reboot.53Btrfs subvolumes are an ideal solution for persistent data. They provide a native, efficient, and flexible way to isolate mutable directories like /var, /home, /opt, and /usr/local from the immutable root filesystem.49 This isolation ensures that user data, logs, databases, and manually installed applications persist across OS image updates or rollbacks, which is a fundamental requirement for a "feature complete" immutable system. Furthermore, subvolumes can be snapshotted independently, providing granular data protection and recovery capabilities.49 This isolation of mutable data from the immutable root filesystem ensures data persistence across OS updates and rollbacks, effectively mimicking bootc's separation of mutable state.To truly achieve the "feature complete" goal of a bootc alternative, the adoption of Btrfs with its subvolume and snapshot capabilities is strongly suggested. Without Btrfs, managing persistent data across OS image changes becomes significantly more complex, potentially relying on less atomic methods like rsync for synchronization.54 This implies that to fully realize the benefits of a bootc-like system, the target system's mutable partitions should ideally leverage Btrfs, which introduces a dependency on Btrfs support and familiarity for deployment.The following table outlines a recommended filesystem layout for an immutable Debian/Ubuntu system, integrating the concepts of SquashFS, OverlayFS, and Btrfs subvolumes for robust persistence and immutability.Table 2: Recommended Filesystem Layout for Immutable Debian/UbuntuPartition/SubvolumePurposeFilesystem TypeMount OptionsPersistence/boot/efiUEFI bootloader and configurationFAT32rwPersistent/boot (Optional)Kernel images and initial RAM filesystemext4 (or Btrfs subvolume)ro (or rw if separate)Persistent/ (Root Filesystem)Immutable base operating systemSquashFS (lower) + OverlayFS (upper)ro (SquashFS) + rw (OverlayFS)Ephemeral (OverlayFS upperdir if tmpfs) / Persistent (if OverlayFS upperdir on disk)/varVariable system data (logs, caches, databases, container storage)Btrfs subvolumerwPersistent/homeUser home directoriesBtrfs subvolumerwPersistent/tmpTemporary filestmpfsrwEphemeral/optThird-party applicationsBtrfs subvolumerwPersistent/usr/localLocally installed software/binariesBtrfs subvolumerwPersistent/.snapshotsBtrfs snapshots for system and dataBtrfs subvolumerwPersistentThis table visually summarizes the intricate interactions between different filesystem components and their respective roles in maintaining system integrity and data longevity, directly addressing the "feature complete" requirement by detailing the underlying storage architecture.4. Transactional Updates and Robust RollbacksA hallmark of bootc and immutable operating systems is their ability to perform atomic, transactional updates and provide robust rollback mechanisms. Replicating this functionality in Debian/Ubuntu requires careful consideration of update strategies and bootloader configurations.4.1 Atomic Update Principles for Debian/Ubuntubootc updates are inherently transactional and are performed in-place using container images.1 The update process writes the new operating system content to a new root filesystem, leaving the currently running system completely untouched. The changes only become active after a system reboot into the newly prepared root.10Two primary approaches can achieve atomic updates in Debian/Ubuntu:A/B Partitioning Scheme: This method involves maintaining two complete copies of the root filesystem, typically designated as slot A and slot B. The system operates from one active slot (e.g., A) while updates are written to the inactive slot (B). Upon successful completion of the update to slot B, the bootloader is reconfigured to switch to slot B on the next reboot.12 This provides a highly robust, fail-safe update mechanism, as the previous working slot remains untouched and available for immediate rollback if the new system encounters issues or fails to boot.57 The primary drawback of this scheme is the requirement for approximately double the storage capacity, as two full system images must be maintained.57Btrfs Snapshots with snapper: This alternative offers transactional updates and rollbacks at the filesystem level, making it conceptually closer to bootc's ostree approach in terms of efficiency. snapper leverages Btrfs's copy-on-write capabilities to create snapshots of the root filesystem (and other designated subvolumes) both before and after system modifications.8 Updates are applied to a new snapshot in the background, minimizing disruption to the running system.8 If the update fails or introduces problems, the new snapshot can be discarded, or the system can be rolled back to any previous, known-good snapshot.8 Similar to A/B, changes become active only after a reboot into the selected snapshot.Traditional dpkg/apt package updates in Debian/Ubuntu are generally not atomic. A power outage or system crash during an update can leave the system in an inconsistent or unbootable state, although such issues are often repairable with manual intervention.59The choice between A/B partitioning and Btrfs snapshots with snapper represents different approaches to atomicity. A/B partitioning provides strong isolation by physically separating OS versions, offering a guaranteed fallback.56 This is robust but resource-intensive due to duplicated storage. Btrfs snapshots, while also atomic and enabling rollbacks, are more space-efficient for incremental changes due to copy-on-write.46bootc's ostree is conceptually closer to Btrfs snapshots in its efficiency (content-addressed) but operates at a higher level of abstraction.14 The decision between A/B and Btrfs depends on specific requirements for storage, isolation, and update frequency. For general-purpose Debian/Ubuntu systems (e.g., desktops, smaller servers), Btrfs with snapper is often a more practical and space-efficient choice than full A/B partitioning. A/B is more commonly found in embedded systems or mission-critical appliances where maximum isolation and guaranteed fallback are paramount.A key benefit of bootc and A/B updates is the ability to perform updates in the background while the system is running, with minimal downtime limited to a quick reboot.12snapper also facilitates background updates by applying changes to a new snapshot.8 This contrasts sharply with traditional apt upgrades that modify the live system and can be disruptive.59 Achieving truly non-disruptive updates requires careful orchestration of the update process, but the use of snapshots or A/B schemes significantly reduces user interruption and system downtime, thereby improving system availability and user experience, aligning with bootc's operational benefits.4.2 Bootloader Configuration for RollbackThe bootloader is a fundamental component responsible for initiating the system boot process.16 In bootc's design, it integrates with bootupd to handle bootloader installation and upgrades seamlessly.16 For A/B update schemes, the bootloader's critical role is to select the active partition. Crucially, if the newly updated partition fails to boot, the bootloader must be capable of reverting to the previously working partition to ensure system availability.12GRUB (GNU GRand Unified Bootloader) is the predominant bootloader for both Debian and Ubuntu systems.17 Its flexibility makes it suitable for managing multiple OS versions and rollback points. For systems utilizing Btrfs snapshots, grub-btrfs is a critical tool. It enhances GRUB by dynamically adding a sub-menu that allows users to boot directly into various Btrfs snapshots.63 This transforms GRUB into a powerful "rollback switchboard," providing a user-friendly and robust mechanism for reverting to previous system states directly from the boot menu, mirroring bootc's capability to switch between ostree deployments.10 This significantly enhances system resilience and recovery.To maintain an up-to-date boot menu without manual intervention, grub-btrfs includes a daemon called grub-btrfsd. This daemon automatically updates the GRUB configuration whenever new Btrfs snapshots are created or deleted, ensuring the boot menu always reflects the available system states.66 This automation is crucial for operational efficiency and user experience, especially in a system designed for frequent, atomic updates, as it eliminates the need for manual update-grub commands after every OS image change or snapshot creation.When managing Btrfs snapshots, changing the default subvolume (e.g., to point to a specific snapshot for booting) can be achieved using btrfs subvolume set-default. Following this, the GRUB configuration needs to be updated to ensure it respects this new default and does not override it with hardcoded kernel arguments.52 This ensures that the system boots into the intended snapshot.5. Building and Deploying Bootable Container ImagesThe culmination of an immutable Linux system involves assembling the base image with all necessary components and then packaging it into a format suitable for bootc-like deployment.5.1 Image Assembly with live-buildlive-build is the primary tool for creating custom Debian Live system images, encompassing ISOs, netboot images, and USB stick images.20 Its highly configurable nature, managed through a dedicated config/ directory, allows for precise control over the image's contents and structure.21The live-build process is structured into distinct stages:bootstrap stage: This initial phase populates the chroot directory with a barebones Debian system, typically using debootstrap.32chroot stage: This is where the primary customization occurs. Packages are added, local files are copied, and custom hook scripts are executed to configure the system within the chroot environment.32binary stage: In this final stage, the tool generates the bootable image. This often involves creating a SquashFS root filesystem, which serves as the immutable base, and including the installer and any additional materials on the target medium.32live-build supports the integration of custom kernels 31, enabling the inclusion of specialized kernels tailored for specific hardware or features. It can also be configured to utilize OverlayFS, providing a writable layer on top of the read-only SquashFS base, which is fundamental for achieving the desired immutability behavior.34 The tool can generate various output formats, including iso, iso-hybrid, netboot, tar, and hdd images.72 These outputs can then be converted to or directly used as virtual machine disk images like QCOW2.73live-build functions as the "Containerfile" for bootable images. Analogous to how a Containerfile (Dockerfile) declaratively defines an OCI application image, live-build's configuration (through its config/ directory, lb config options, and hook scripts) declaratively specifies the contents and structure of the bootable OS image.21 This declarative approach enables version control and reproducible builds of the entire operating system, aligning perfectly with the GitOps and CI/CD principles emphasized by bootc.1 This declarative approach to OS image building is a cornerstone of the bootc philosophy, facilitating automated, repeatable, and auditable OS image creation and management.While live-build is primarily designed for "live" systems that boot from removable media and are often ephemeral 20, transforming this into a persistent, immutable appliance akin to bootc requires careful configuration. The live-build output must be configured to use a persistent writable layer (e.g., an OverlayFS upperdir on a dedicated disk partition, or Btrfs subvolumes for /etc and /var) rather than merely a tmpfs.36 This careful integration of persistence mechanisms transforms the ephemeral live system into a bootable, immutable appliance with persistent state.5.2 Packaging as OCI Imagesbootc's core innovation lies in its use of standard OCI/Docker containers as the primary format for operating system transport and delivery.1 To achieve a bootc-like alternative, the final step involves packaging the assembled Debian/Ubuntu image into an OCI-compliant format.Tools like podman save can be used to save an existing container image to a local file or directory in various formats, including oci-dir, which represents an OCI Image Format as a directory structure.76 For transferring these images, skopeo copy is a versatile utility capable of copying container images between different locations, including from local storage to remote registries.78 An OCI image fundamentally consists of a set of filesystem layers and a configuration blob, defining the image's characteristics and runtime behavior.79The final step involves OCI packaging for distribution. This means converting the assembled Debian/Ubuntu image into an OCI artifact for bootc-like distribution and management. While podman save can create OCI-compliant directories, the challenge lies in ensuring that this OCI image is truly "bootable" in the bootc sense. bootc's install command is specifically designed to bridge the gap between a runnable OCI image and a bootable system by executing embedded tooling logic.16 This implies that a custom Debian/Ubuntu solution would either need to replicate this bootc install functionality or integrate with bootc's podman-bootc package for the final bootable aspect from the OCI image. The difference between packaging an application container and a full OS image is significant, especially regarding the inclusion and proper integration of the kernel and bootloader within the OCI layers, and how the system is initiated from this image.6. Conclusion and RecommendationsThe development of a feature-complete bootc-inspired immutable Linux system for Debian/Ubuntu is a complex yet achievable endeavor. It requires a thoughtful integration of existing, robust tools to replicate the benefits of bootc's ostree-based approach without directly adopting ostree itself.The analysis indicates that the shift from package-centric to image-centric OS management is fundamental for achieving predictable, consistent, and secure deployments. By treating the OS as an atomic unit, the system's state becomes more manageable, and security posture is enhanced through comprehensive image scanning and a read-only root filesystem. The transactional update model, analogous to a database transaction, is crucial for system reliability and minimizing downtime by ensuring updates either fully succeed or leave the system unchanged.A key challenge identified is the sophisticated handling of /etc configuration files. While ostree provides an automatic 3-way merge, a Debian/Ubuntu alternative, particularly when relying on OverlayFS, would require custom scripting or external configuration management tools to reconcile local changes with upstream updates. This represents a functional gap that demands careful design and implementation to avoid manual intervention and ensure scalability.For managing persistent data in /var and /home, the adoption of Btrfs with its subvolume and snapshot capabilities is highly recommended. Btrfs subvolumes offer an efficient and flexible way to isolate mutable data from the immutable root, ensuring data longevity across OS updates and providing granular recovery points. The integration of snapper with Btrfs further enables atomic updates and robust rollbacks, significantly enhancing system resilience.The initramfs emerges as a critical orchestrator, responsible for setting up the immutable root filesystem during early boot. Customizing the kernel to include necessary drivers and filesystem features (like SquashFS and OverlayFS) as built-in components is essential for a minimal and optimized image.Finally, live-build serves as a powerful "Containerfile" for declaratively assembling the bootable OS image, enabling reproducible builds. The ultimate step involves packaging this assembled image into an OCI-compliant format, though the final "bootable" aspect from the OCI image would require custom tooling or integration with bootc's podman-bootc to bridge the gap between a standard container image and a bootable system.Recommendations for Implementation:Adopt Btrfs as the Primary Filesystem: For the target system, configure Btrfs for the root filesystem and separate subvolumes for /etc, /var, /home, /opt, and /usr/local. This provides the foundation for persistence, atomic updates, and efficient snapshots.Integrate snapper for Transactional Updates: Install and configure snapper to create pre- and post-snapshots around all system modifications (e.g., package installations, configuration changes). This enables atomic updates and provides a robust rollback mechanism.Leverage grub-btrfs for Rollback Management: Install grub-btrfs and enable its daemon (grub-btrfsd) to automatically populate the GRUB boot menu with Btrfs snapshots. This transforms GRUB into a user-friendly "rollback switchboard," allowing for easy reversion to previous system states.Utilize live-build for Image Assembly: Employ live-build with a declarative configuration (via its config/ directory and hooks) to define the base OS image. Configure it to use a SquashFS for the read-only core and integrate OverlayFS for the writable layer, ensuring persistence for the /etc and /var overlays on dedicated Btrfs subvolumes.Develop Custom Kernel and Initramfs: Build a custom Linux kernel tailored to target hardware, ensuring necessary filesystem drivers (SquashFS, OverlayFS, Btrfs) are built-in. Develop a custom initramfs script to orchestrate the mounting of the read-only SquashFS base and the persistent OverlayFS upperdir during early boot.Automate OCI Packaging and Distribution: After image assembly, use podman save to convert the disk image into an OCI-compliant directory structure. Develop custom scripts or integrate with bootc's podman-bootc to make this OCI image directly bootable, enabling distribution via standard container registries.Address /etc Merge Complexity: Implement a strategy for /etc configuration management. This could involve:Declarative Configuration Management: Using tools like Ansible or Puppet to apply configurations after each update, treating /etc as a generated state.Custom Scripting for Merge: Developing custom scripts that analyze snapper diffs for /etc and attempt to merge changes, potentially requiring manual conflict resolution.Strict Policy: Minimizing local modifications to /etc and relying primarily on upstream defaults, with critical local changes managed via a separate, version-controlled overlay.By meticulously following these recommendations, a Debian/Ubuntu system can achieve a high degree of immutability, transactional integrity, and simplified management, closely mirroring the capabilities and benefits offered by bootc.