Why Microsoft Azure Now Relies on Linux: Inside the Cloud’s Massive Linux Shift

Once a Windows‑only cloud host, Microsoft Azure now tests more than 1,000 Linux distributions each month to ensure smooth operation of customer applications.

In 2001, then‑CEO Steve Ballmer dismissed Linux as a "cancer," but two decades later Linux has become the most widely used operating system on Azure, surpassing Windows.

Azure Linux Platform managers Jack Aboutboul and Krum Kashan explained at the Linux Foundation Open Source Summit that Microsoft now invests heavily to make Linux run as smoothly as possible on Azure, with Linux accounting for over 60% of Azure market products and virtual‑machine kernels.

"We originally used Windows, but now Linux is the #1 operating system on Azure," – Jack Aboutboul.

To support this, Microsoft’s Linux team provides system‑level support for both internal and Azure customers, and has built its own Azure‑optimized Linux kernel released in 2023.

Linux Guest OS Development Lifecycle

Azure receives roughly 1,000 images per month from approved partners, with many distributions offering multiple images (e.g., standard and high‑performance variants). The team faces scheduling challenges because each distribution follows its own release cadence, and some, like Oracle Unbreakable Linux, have no fixed schedule.

When a new distribution is launched, Azure tests updates before they reach customers to avoid breaking systems, and aims to standardize security patches through the Azure Guest Patching Service (AzGPS).

Customers can run any Linux they wish on Azure, import their own images, or build custom VHDs, with Microsoft striving to support even niche cases such as Gentoo.

How Microsoft Tests Linux on Azure

Microsoft continuously monitors upstream kernels, builds its own Azure‑tuned kernel every 12 hours, and feeds validation results back to the Linux kernel community.

The Azure Linux Fleet runs millions of VMs across thousands of SKUs, performing about 5 million tests per month covering performance, functionality, stress, and community testing.

To automate testing, Microsoft developed the LISA (Linux Integration Services Automation) framework, with many Python‑based test cases released as open source.

https://mslisa.readthedocs.io/en/main/run_test/microsoft_tests.html

Microsoft plans to expose LISA as a self‑service portal called Azure Image Testing Service for Linux (AITL), allowing third parties to validate their Linux images via API or GUI, with results kept confidential for integration into CI/CD pipelines.

Author: Luo Yi

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