Background

Historically, Linux distributions have utilized package managers for both initial installation and ongoing updates. While this approach offers flexibility for individual users, it presents challenges for large-scale deployments where service dependencies are well defined. Image-based deployment and servicing provide a more robust solution, enabling atomic updates, reliable rollback, and reduced downtime. Trident introduces a unified, declarative workflow for installation and servicing of Linux distributions—particularly Azure Linux—by leveraging image-based A/B style updates to ensure consistency, reliability, and operational efficiency.

Installer image vs. VM image

Linux distributions typically provide two primary types of bootable images: an installer image and a virtual machine (VM) image.

The installer image, typically distributed in ISO format, boots into a live servicing operating system and can be executed in either attended or unattended modes. During installation, users are able to configure disk partitioning, select desired features, and set system parameters such as timezone and user accounts. Behind the scenes, the installer automates essential tasks including disk partitioning and formatting, package installation, and operating system configuration. Upon completion, the installer reboots into the fully provisioned runtime environment. This installation process is suitable for deploying Linux distributions to both bare-metal hosts and virtual machines.

Alternatively, if your goal is to run the Linux distribution within a virtual machine, you can obtain a VM image and initiate the boot process immediately. This approach enables rapid deployment and operation of the Linux environment. However, configuration changes can only be applied after the initial boot—typically using tools such as cloud-init—which may necessitate additional reboots to fully realize the desired system state.

The need for ongoing servicing

Regardless of the image type selected, ongoing servicing is essential to address security vulnerabilities (CVEs) and apply updates. For environments with spare resources, scale-out servicing can be performed by deploying a new OS version onto a separate node—virtual or bare-metal—and decommissioning the older instance upon completion. However, this approach may not be feasible for larger clusters or resource-constrained scenarios where spare capacity is unavailable or cost-prohibitive.

In such cases, servicing can be achieved by shutting down the current OS instance, replacing the OS disk (either by deploying a new VM image or rerunning the installer), and booting into the updated OS. This process is time-consuming and requires additional orchestration from the underlying infrastructure.

The advantages of in-place servicing

To minimize downtime and avoid reliance on spare resources, in-place servicing is preferable. Traditional Linux distributions typically support package-based updates, but these methods lack robust rollback capabilities and can result in inconsistencies across nodes due to timing and package variations.

A more reliable approach is to use image-based A/B style in-place atomic updates, similar to those used by Android. With A/B updates, rollback is straightforward—either during servicing or at any later point—without requiring extra resources. Additionally, servicing downtime is reduced, as the B set of images can be pre-staged while the A set remains operational.

The Trident solution

Traditionally, Linux distributions provide distinct mechanisms for initial installation and subsequent servicing of the operating system. Trident streamlines this process by offering a unified workflow that seamlessly handles both installation and ongoing servicing tasks.

Regardless of whether you are deploying to bare-metal hosts or virtual machines, and whether you utilize installer images or VM images, Trident delivers a consistent, atomic approach to OS deployment and servicing. Its composable architecture enables easy integration into broader solutions, eliminating the need to manually coordinate low-level OS utilities for disk partitioning, image installation, bootloader configuration, and system setup—Trident manages these operations efficiently on your behalf.