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Initramfs: Fixing Kernel Boot Issues with Initramfs

Problem

I just installed a server with many SATA disks. The machine has 4.5TB of storage spread across 2 Areca ARC-1280ML controllers. The Debian/etch installation went without issues using kernel 2.6.18-5-686. After installing the system on 2 disks connected to the motherboard (ICH5R controller using the ata_piix driver), RAID5 volumes are created on the Areca cards (arcmsr driver). However, boot stops at an initramfs command prompt, unable to find the root partition:

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Begin: Waiting for root file system... ...
Done.
        Check root= bootarg cat /proc/cmdline
        or missing modules, devices: cat /proc/modules ls /dev
ALERT! /dev/sda1 does not exist. Dropping to a shell!


Busybox v1.1.3 (Debian 1:1.1.1-4) Built-in shell (ash)
Enter 'help' for a list of built-in commands.

/bin/sh: can't access tty; job control turned off
(initramfs)

Explanation

What's happening? Simply put, the new volumes are detected by the kernel before the disk on which the system is installed. As a result, the system is no longer on /dev/sda but on /dev/sdc. And the funniest part is that it's sometimes on /dev/sdb because the Areca controllers take time to initialize.

How do we fix this issue? By working with the initialization RAM partition, namely initramfs.

Solution

It's extremely simple. We'll ask the RAM boot partition to load the SATA modules in the order we want. In our case, the ata_piix driver before arcmsr. Debian tools make this very easy, just add the modules you want loaded during startup to the /etc/initramfs-tools/modules file. Modules should be listed one per line in the desired loading order. In our case, we just need to specify the module that handles the boot disk.

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# cat /etc/initramfs-tools/modules
[...]
ata_piix

Now we need to update the RAM image to apply these changes:

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# update-initramfs -v -k 2.6.18-5-686 -t -u
Keeping /boot/initrd.img-2.6.18-5-686.dpkg-bak
update-initramfs: Generating /boot/initrd.img-2.6.18-5-686
Adding module /lib/modules/2.6.18-5-686/kernel/drivers/scsi/scsi_mod.ko
Adding module /lib/modules/2.6.18-5-686/kernel/drivers/scsi/scsi_transport_spi.ko
Adding module /lib/modules/2.6.18-5-686/kernel/drivers/scsi/aic7xxx/aic7xxx.ko
[...]
Adding binary /sbin/mdrun
Building cpio /boot/initrd.img-2.6.18-5-686 initramfs
Backup /boot/initrd.img-2.6.18-5-686.bak

After a reboot, everything is back to normal. The best part is that when you need to update your kernel, the new kernel will automatically rebuild the initialization RAM image.