120 lines
3.9 KiB
Markdown
120 lines
3.9 KiB
Markdown
---
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title: "The Case of the Bloated Boot"
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date: 2025-09-22
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lastmod: 2025-09-22
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categories: ["Blog"]
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tags: ["linux"]
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---
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When `sudo mkinitcpio -P` fails because your boot partition is full, you've got
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a fun problem. The culprit? That massive fallback initramfs taking up precious
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space on a tiny boot partition.
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<!--more-->
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# The Problem
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Picture this: you're dual-booting Linux and running a system update when
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suddenly `mkinitcpio` fails to generate the larger fallback image. Your boot
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partition is completely stuffed.
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```bash
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$ df -h /boot
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Filesystem Size Used Avail Use% Mounted on
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/dev/nvme0n1p1 256M 256M 0 100% /boot
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```
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**Yikes.** Time to investigate what's eating all that space.
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# Damage Assessment
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A quick `du -sh` reveals the usual suspects:
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```bash
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$ du -sh /boot/*
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157M /boot/initramfs-linux-fallback.img
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52M /boot/initramfs-linux.img
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16M /boot/vmlinuz-linux
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30M /boot/EFI
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```
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The fallback initramfs is **three times larger** than the normal one. But why?
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# The Autodetect Mystery
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The secret lies in one simple flag in `/etc/mkinitcpio.d/linux.preset`:
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```bash
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fallback_options="-S autodetect"
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```
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That `-S autodetect` flag skips the autodetect hook, which normally shrinks your
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initramfs by only including modules your hardware actually needs. Without it,
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mkinitcpio includes **everything**.
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## By the Numbers
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Here's what that means in practice for me:
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| Component | Normal (with autodetect) | Fallback (no autodetect) | Ratio |
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| :----------------- | -----------------------: | -----------------------: | ------: |
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| **Kernel modules** | 190 modules | 6,319 modules | **33x** |
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| **Firmware files** | 711 files | 2,996 files | **4x** |
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| **Total size** | 52M | 157M | **3x** |
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The fallback image is essentially a universal boot disk that works on _any_
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hardware — your laptop, your friend's desktop, that ancient server in the
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closet. It includes drivers for RAID controllers you don't have, network cards
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from 2003, and GPU firmware for every generation of graphics hardware.
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# The Nuclear Option
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Since I never use the fallback image anyway (live USB for rescue), the
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solution was just to get rid of this:
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1. **Remove the fallback image** to free immediate space:
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```bash
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sudo rm /boot/initramfs-linux-fallback.img
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```
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2. **Disable future generation** by editing `/etc/mkinitcpio.d/linux.preset`:
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```bash
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# Change this line:
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PRESETS=('default' 'fallback')
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# To this:
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PRESETS=('default')
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```
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3. **Test it works**:
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```bash
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sudo mkinitcpio -P
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```
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Result: boot partition goes from 100% to 39% usage, and future kernel updates
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won't recreate the bloated fallback.
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# Why It Matters
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The autodetect hook is actually quite clever. On my system with 443MB of total
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kernel modules available, it correctly identified that I only need 190 of them
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— things like my specific WiFi driver, filesystem modules for ext4 and LVM, and
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encryption support for LUKS.
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The fallback includes _everything else too_: drivers for hardware I'll never own
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(_especially since this is on a laptop_), filesystem support for formats I'll
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never use, and network protocols from the dawn of time. It's the digital
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equivalent of packing for a trip by bringing your entire garage of tools "just
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in case"
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# The Takeaway
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Unless you're regularly swapping hardware or need guaranteed boot recovery from
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the initramfs itself, the fallback image is dead weight. Modern autodetection
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works well enough that most people never need it. I've personnally never, ever
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used the fallback image and for the context of this laptop probably never will
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as a bootable USB is my go-to for any and all boot issues.
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For those tight boot partitions in dual-boot setups, disabling fallback
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generation is it. Save the space for kernel updates that actually matter :^)
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And if you _do_ need recovery? Well, that's what live USBs are for, but you're
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welcome to make your own decisions. |