The Cloud-Init Gotcha: How a Single GRUB Override Derailed an 8-GPU Debugging Session

Introduction

In the high-stakes world of large-scale GPU inference deployment, few things are more frustrating than a fix that should work but doesn't. Message [msg 352] captures one such moment with surgical precision. After an extensive session of debugging PCIe BAR allocation failures that left 6 of 8 NVIDIA RTX PRO 6000 GPUs undetected in a Proxmox virtual machine, the assistant applies what seems like the obvious fix—adding pci=realloc to the guest kernel command line—only to find it silently ignored. The message is a masterclass in diagnostic reasoning: a single observation, a hypothesis about configuration ordering, and a targeted verification that uncovers the root cause.

The Broader Context: A BAR-Sized Problem

To understand the significance of this message, one must appreciate the preceding struggle. The team was deploying a GLM-5-NVFP4 large language model across 8 Blackwell-generation GPUs, each with 96GB of VRAM. After migrating the VM from the legacy i440FX chipset to Q35 with pcie=1—a necessary step for proper PCIe passthrough and potential peer-to-peer (P2P) DMA—the VM could only detect 2 of its 8 GPUs. The culprit was a BAR (Base Address Register) allocation failure: each GPU's BAR2 region, which maps its VRAM into the PCIe address space, requires a contiguous 128GB block. With 8 GPUs, that's 1TB of BAR2 space alone, and the Q35 firmware's 64-bit MMIO aperture was insufficient to accommodate all of them.

The guest kernel itself offered the solution in its own error messages. The dmesg output explicitly stated: "Some PCI device resources are unassigned, try booting with pci=realloc". This kernel parameter tells the Linux PCI subsystem to aggressively reassign BAR resources that the firmware failed to allocate. It was the least invasive, most promising fix available.

The Initial Fix and Its Silent Failure

In [msg 344], the assistant dutifully modified /etc/default/grub on the guest VM, changing GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pci=realloc". It then ran sudo update-grub in [msg 345], which completed successfully, and rebooted the VM in [msg 346]. Everything appeared to have gone correctly.

But when the VM came back up and the assistant checked /proc/cmdline in [msg 351], the pci=realloc parameter was conspicuously absent. The boot command line read:

BOOT_IMAGE=/vmlinuz-6.8.0-100-generic root=UUID=e4cd6da8-09cb-4378-9577-e2053726667f ro console=tty1 console=ttyS0

No pci=realloc. The fix had been applied, update-grub had run without error, yet the kernel booted without the parameter. This is the moment that sets the stage for message [msg 352].

The Diagnostic Insight

Message [msg 352] opens with a concise statement of the problem and its hypothesized root cause:

pci=realloc didn't make it into the cmdline — the 50-cloudimg-settings.cfg sourced after /etc/default/grub is likely overriding GRUB_CMDLINE_LINUX_DEFAULT. Let me check and fix that.

This single sentence reveals the assistant's reasoning process. The key clue was hidden in the update-grub output from [msg 345], which showed:

Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50-cloudimg-settings.cfg'

The order matters. GRUB's configuration system sources files from /etc/default/grub.d/ after the main /etc/default/grub file. If any file in that directory sets the same variable, it will override the main file's value. The assistant recognized this pattern immediately—a sign of deep familiarity with Ubuntu's boot configuration mechanics.

The assistant then issues a bash tool call to read the suspect file, confirming the hypothesis. The contents of /etc/default/grub.d/50-cloudimg-settings.cfg reveal the smoking gun:

# Set the default commandline
GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0"

This file, generated by the Ubuntu cloud image build process, unconditionally sets GRUB_CMDLINE_LINUX_DEFAULT to "console=tty1 console=ttyS0". Because it is sourced after the main /etc/default/grub, it overwrites whatever was set there—including the carefully added pci=realloc.

The GRUB Configuration Chain: A Subtle but Critical Detail

The GRUB configuration system on Ubuntu is not a single file but a layered chain. The boot process works as follows:

  1. /etc/default/grub — The primary configuration file, intended for user modifications.
  2. /etc/default/grub.d/*.cfg — Supplemental configuration files, sourced in lexicographic order after the main file. These are typically installed by packages or cloud image build processes.
  3. update-grub — A script that reads the above files and generates /boot/grub/grub.cfg, the actual configuration used by the GRUB bootloader. The critical detail is that variables set in grub.d/ files override those in the main file because they are sourced later in the same shell environment. This is not a bug—it is a deliberate design that allows packages and cloud images to set defaults that users can override. However, the override mechanism is not intuitive: to override a grub.d/ setting, the user must either modify the grub.d/ file itself or use a different variable (like appending to GRUB_CMDLINE_LINUX instead of GRUB_CMDLINE_LINUX_DEFAULT).

Why This Matters Beyond This Session

This message illustrates a broader class of problems that plague infrastructure debugging: the silent override. The assistant's initial fix was technically correct—modifying /etc/default/grub and running update-grub is the standard procedure on Ubuntu. But the presence of the cloud-init override file created a situation where the standard procedure silently failed. There was no error message, no warning from update-grub, no indication that the modification had been overridden. Only by inspecting the actual boot command line via /proc/cmdline could the discrepancy be detected.

This is a recurring theme in systems engineering: the gap between what you intend to configure and what the system actually uses. Cloud images, in particular, are notorious for such surprises. They are optimized for automated deployment at scale, which means they often include configuration files that override user settings in subtle ways. The 50-cloudimg-settings.cfg file is a prime example: it sets console parameters for headless cloud servers, but in doing so, it inadvertently clobbers any user modifications to the same variable.

The Thinking Process Revealed

The assistant's reasoning in this message demonstrates several hallmarks of expert debugging:

  1. Observation: The pci=realloc parameter is missing from /proc/cmdline despite being added to GRUB config.
  2. Recall: The update-grub output showed two sourcing events: the main file and a grub.d/ file.
  3. Hypothesis: The grub.d/ file likely sets the same variable, overriding the main file.
  4. Verification: Read the suspect file to confirm.
  5. Action: The message ends with "Let me check and fix that," signaling the next step. Notably, the assistant does not panic or reapply the same fix. It does not assume a transient error or a reboot issue. It traces the problem to its logical source based on the available evidence. This is the essence of systematic debugging: following the chain of cause and effect rather than guessing.

What Comes Next

The natural follow-up to this discovery is to modify the cloud-init override file instead of (or in addition to) the main GRUB file. The assistant could either:

Broader Lessons

Message [msg 352] teaches several important lessons for anyone working with Linux systems, virtualization, or GPU infrastructure:

  1. Verify your fixes: Always check that a configuration change actually took effect by inspecting the runtime state (/proc/cmdline, dmesg, etc.), not just the configuration file.
  2. Understand the configuration chain: GRUB's grub.d/ directory, sysctl's .conf files, systemd's drop-in directories—Linux is full of layered configuration systems where order matters.
  3. Cloud images are special: Systems built from cloud images (AWS, Proxmox templates, etc.) often include configuration files that differ from a standard installation. Never assume a standard procedure will work unchanged.
  4. Read the full output: The clue was in the update-grub output, which showed both sourcing events. A less careful operator might have glossed over that detail.
  5. Follow the evidence chain: The assistant traced the problem from symptom (missing parameter) to configuration (GRUB file) to override (cloud-init file) in a clean, logical chain.

Conclusion

Message [msg 352] is a small but perfectly formed debugging vignette. In just a few lines of reasoning and a single tool call, the assistant identifies why a seemingly correct fix failed, pinpoints the obscure mechanism responsible, and sets up the next corrective action. It is a reminder that in complex systems, the most insidious failures are often not the catastrophic ones but the silent overrides—the configuration that was applied but never took effect, the parameter that was set but never read. The ability to recognize these patterns and trace them to their source is what separates effective debugging from trial-and-error guesswork.