The Cloud-Init Override: A Lesson in Ubuntu Boot Configuration

Introduction

In the course of deploying a large language model inference server across eight NVIDIA RTX PRO 6000 Blackwell GPUs in a Proxmox virtual machine, a seemingly simple fix—adding pci=realloc to the Linux kernel command line—failed silently. The assistant had carefully diagnosed a PCI BAR allocation problem that prevented six of eight GPUs from being detected, and the guest kernel itself had suggested the solution: "try booting with pci=realloc." Yet after editing the GRUB configuration, running update-grub, and rebooting, the parameter was absent from the running kernel's command line. Message 353 in this conversation records the moment of discovery: a cloud-init configuration file was silently overriding the GRUB settings, and the assistant identified and corrected the root cause.

This article examines that single message—its reasoning, context, assumptions, and the knowledge it both consumed and produced—as a case study in the hidden complexities of Linux boot configuration in cloud-provisioned virtual machines.

Context: The PCI BAR Allocation Crisis

To understand message 353, one must first understand the crisis that preceded it. The user had migrated their Proxmox VM from the legacy i440FX chipset to the modern Q35 chipset with PCIe passthrough enabled (pcie=1), a necessary step toward enabling peer-to-peer DMA communication between GPUs. This migration exposed a severe resource allocation problem: each RTX PRO 6000 GPU requires a 128GB Base Address Register 2 (BAR2) to map its VRAM into the PCI address space. With eight GPUs, that is 1TB of BAR2 space alone, plus additional BARs for control registers.

The guest firmware (SeaBIOS) and kernel could only allocate BAR2 for two of the eight GPUs. The remaining six failed with errors like:

NVRM: BAR2 is 0M @ 0x0 (PCI:0000:01:00.0)
nvidia: probe of 0000:01:00.0 failed with error -1

The kernel itself suggested the fix: "Some PCI device resources are unassigned, try booting with pci=realloc." The assistant recommended this as Option 2 in its plan, and the user agreed: "apply 2 to the guest."

The Failed Fix

In messages 344–346, the assistant executed the fix: it SSHed into the VM (llm-one), edited /etc/default/grub to change GRUB_CMDLINE_LINUX_DEFAULT from "quiet splash" to "quiet splash pci=realloc", ran sudo update-grub, and rebooted. The commands succeeded without error. Yet when the VM came back up (message 351), nvidia-smi -L still showed only two GPUs, and cat /proc/cmdline revealed:

BOOT_IMAGE=/vmlinuz-6.8.0-100-generic root=UUID=e4cd6da8-... ro console=tty1 console=ttyS0

No pci=realloc. The parameter was simply absent.

The Discovery: Message 353

Message 353 is the assistant's response to this failure. It opens with a moment of recognition: "There it is — this cloud-init file overrides GRUB_CMDLINE_LINUX_DEFAULT after /etc/default/grub is sourced." The assistant had checked /etc/default/grub.d/50-cloudimg-settings.cfg (in the preceding message, 352) and found the culprit. The cloud-init configuration file contained:

GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0"

Because GRUB processes configuration files in alphabetical order within the /etc/default/grub.d/ directory, and 50-cloudimg-settings.cfg is sourced after the main /etc/default/grub file, the cloud-init file's assignment of GRUB_CMDLINE_LINUX_DEFAULT overwrote the value the assistant had set. The pci=realloc parameter was lost.

The assistant then executed the fix: it used sed to edit the cloud-init file, adding pci=realloc to the existing console=tty1 console=ttyS0 string, producing:

GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0 pci=realloc"

The message also notes an alternative approach: using GRUB_CMDLINE_LINUX (as opposed to GRUB_CMDLINE_LINUX_DEFAULT), which "gets appended and isn't overridden." This is a key insight into GRUB's variable semantics.

The Reasoning and Thinking Process

The assistant's reasoning in this message reveals several layers of diagnostic thinking:

First, pattern recognition. The assistant immediately recognized that the cloud-init file was the source of the override. This required knowledge of Ubuntu's GRUB configuration hierarchy: the main /etc/default/grub file is processed first, then files in /etc/default/grub.d/ are sourced in alphabetical order. Cloud-init images for Ubuntu (especially those used in cloud/VPS environments) commonly include a 50-cloudimg-settings.cfg file that sets console parameters. The assistant had seen this pattern before.

Second, causal inference. The assistant connected the absence of pci=realloc in /proc/cmdline to the presence of the cloud-init override. This required understanding that GRUB_CMDLINE_LINUX_DEFAULT is a variable, not an accumulator—the last assignment wins. When 50-cloudimg-settings.cfg reassigns it, the earlier value from /etc/default/grub is lost.

Third, solution design. The assistant offered two approaches: edit the cloud-init file (the chosen path) or use GRUB_CMDLINE_LINUX instead. The latter is a more robust approach because GRUB_CMDLINE_LINUX is appended to the kernel command line after GRUB_CMDLINE_LINUX_DEFAULT and is not typically overridden by cloud-init files. However, editing the cloud-init file was the more direct fix for the immediate problem.

Assumptions Made

The assistant made several assumptions in this message, most of which were correct but worth examining:

Assumption 1: The cloud-init file was the sole cause. The assistant assumed that the cloud-init override was the only reason pci=realloc was missing. This was correct—the edited /etc/default/grub value was indeed overwritten. However, there was a secondary assumption that update-grub had been run successfully after the first edit, which it had (message 345).

Assumption 2: The user's VM was cloud-provisioned. The presence of 50-cloudimg-settings.cfg strongly suggests the VM was created from a cloud image (e.g., an Ubuntu cloud image downloaded and used as the VM disk). This is a common practice in Proxmox environments. The assistant assumed this context without explicitly verifying it.

Assumption 3: The sed command would work correctly. The assistant used a sed substitution that matched the exact string GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0". If the file had contained different whitespace or formatting, the substitution would have failed silently. In this case, it succeeded.

Assumption 4: The user had sudo access. The assistant used sudo sed to edit the file, assuming the user had passwordless sudo configured. This was correct, as previous commands had used sudo successfully.

Mistakes and Incorrect Assumptions

While the assistant's diagnosis was correct, there are several points worth critiquing:

Mistake 1: Not checking for override files initially. When the assistant first edited /etc/default/grub in message 344, it did not check whether any GRUB override files existed in /etc/default/grub.d/. A more thorough approach would have been to inspect the entire GRUB configuration directory before making changes. The assistant assumed that editing the main configuration file would be sufficient, which is a reasonable assumption on non-cloud systems but fails on cloud-provisioned images.

Mistake 2: Not verifying the fix before rebooting. After running update-grub, the assistant could have checked the generated GRUB configuration in /boot/grub/grub.cfg to verify that pci=realloc appeared in the kernel command line. This would have caught the override before the reboot, saving time and avoiding the confusion of the VM coming back with only two GPUs.

Mistake 3: The assumption about GRUB variable semantics. While the assistant correctly identified that GRUB_CMDLINE_LINUX_DEFAULT was being overridden, the suggestion to use GRUB_CMDLINE_LINUX as an alternative is not entirely robust. The cloud-init file could also override GRUB_CMDLINE_LINUX if it chose to. The truly robust approach would be to either (a) remove or disable the cloud-init override file, (b) use a higher-priority file in /etc/default/grub.d/ (e.g., 99-local.cfg), or (c) modify the cloud-init file as was done. The assistant chose option (c), which was the most practical.

Input Knowledge Required

To understand and produce message 353, the assistant required substantial domain knowledge:

GRUB configuration hierarchy. Understanding that /etc/default/grub is the main configuration file, that files in /etc/default/grub.d/ are sourced in alphabetical order, and that variable assignments are not additive—the last assignment wins.

Ubuntu cloud image conventions. Knowing that cloud-provisioned Ubuntu images include a 50-cloudimg-settings.cfg file that sets console parameters. This is a common source of confusion for users accustomed to bare-metal or manually-installed Ubuntu systems.

GRUB variable semantics. Understanding the difference between GRUB_CMDLINE_LINUX_DEFAULT (parameters added to the kernel command line for normal boots) and GRUB_CMDLINE_LINUX (parameters added for all boots, including recovery). The assistant noted that GRUB_CMDLINE_LINUX "gets appended and isn't overridden," which reflects an understanding that cloud-init files typically only override the _DEFAULT variant.

Linux kernel PCI resource management. Understanding what pci=realloc does—it tells the kernel's PCI subsystem to attempt to reassign unassigned BARs during boot, rather than leaving them unassigned as the firmware left them.

Sed command syntax. The assistant used sed -i for in-place editing with a substitution pattern, a standard but nontrivial skill.

Proxmox and QEMU virtualization. Understanding the broader context of PCI passthrough, BAR allocation, and the Q35 chipset was necessary to even know that pci=realloc was the right fix.

Output Knowledge Created

Message 353 produced several forms of knowledge:

A corrected GRUB configuration. The immediate output was the edited 50-cloudimg-settings.cfg file with pci=realloc appended to the kernel command line. This was the prerequisite for the subsequent reboot that would (hopefully) enable all eight GPUs.

A documented debugging pattern. The message documents the process of discovering that a cloud-init override was the cause of the failed fix. This pattern—check /proc/cmdline to verify parameters, then trace back through GRUB's configuration chain—is reusable for any similar issue.

An explicit note about GRUB variable semantics. The assistant's comment about GRUB_CMDLINE_LINUX being appended and not overridden is a piece of knowledge that the user (and any reader of the conversation) can apply to future GRUB configuration problems.

A cautionary tale about cloud images. The message implicitly teaches that cloud-provisioned VMs have configuration quirks that bare-metal installations do not. The 50-cloudimg-settings.cfg file is a common trap, and knowing about it saves debugging time.

The Broader Significance

Message 353 is a small but pivotal moment in a much larger engineering effort. The user was attempting to deploy a 400-billion-parameter language model (GLM-5-NVFP4) across eight top-tier GPUs in a virtualized environment—a task that pushes the boundaries of what virtualization can support. The PCI BAR allocation problem was a critical bottleneck, and the pci=realloc fix was the key to unlocking it.

The fact that a cloud-init configuration file—a piece of automation intended to simplify deployment—was silently sabotaging the fix is emblematic of the challenges in modern infrastructure engineering. Layers of abstraction (cloud images, GRUB, firmware, virtualization) interact in ways that no single engineer can fully anticipate. Debugging requires not just technical skill but a systematic approach: verify each layer, check assumptions, and trace the chain of causality from the symptom to the root cause.

The assistant's approach in message 353 exemplifies this systematic thinking. When the expected fix failed, it did not assume the fix was wrong. Instead, it verified the kernel command line, traced back through the GRUB configuration chain, found the override, and corrected it. The message is short—just two sentences of explanation and a single bash command—but it encodes a sophisticated diagnostic process.

Conclusion

Message 353 captures the moment when a hidden configuration override was discovered and corrected. The assistant's reasoning—recognizing the cloud-init pattern, understanding GRUB's configuration hierarchy, and choosing the appropriate fix—demonstrates the depth of knowledge required to debug modern virtualized infrastructure. The message also reveals the assumptions that were made (that editing the main GRUB file would be sufficient) and the mistakes that were avoided (the assistant could have rebooted again without checking, wasting more time).

For the broader narrative of this coding session, message 353 is the turning point in the PCI BAR allocation saga. With the corrected kernel command line, the next reboot would (as we see in subsequent messages) successfully bring up all eight GPUs. The cloud-init override was a small obstacle, but it was the last obstacle before the team could proceed to benchmarking and performance tuning. In the grand scheme of deploying a 400B-parameter model, a missing kernel parameter is a minor detail—but in infrastructure engineering, details are everything.