Passing boot parameters to ScaleWay's baremetal C1 instance Linux kernel

Short story

Add tags like these to your server:

KEXEC_KERNEL=http://mirror.scaleway.com/kernel/armv7l-mainline-lts-4.9-4.9.93-rev1/vmlinuz
KEXEC_INITRD=http://mirror.scaleway.com/initrd/uInitrd-Linux-armv7l-v3.14.6
KEXEC_APPEND=vmalloc=512M

Longer story

The ScaleWay's "BareMetal" "C1" instance is a cheap EUR 3 / month cloud infrastructure instance. It has:

• 4 32bit armv7l cores
• 2 GB RAM
• 50 GB network attached storage
• 1 public IP included in the price

ScaleWay offers two lines of servers:

• BareMetal
• VirtualMachines (KVM based)

One important difference between the two is that:

• A VM can only be booted with as much storage as included in its offer
• Bare metal instances support attaching up-to 15x150 GB additional network block device drives ( charged EUR 1/month per 50 GB )

Another important difference is that currently in ScaleWay infrastructure, contra-logically:

• Only VMs can run custom kernels
• Bare metal servers come with e pre-build kernels and ScaleWay does not officially support changing these kernels. You can't even run the official kernel that comes with the chosen Linux distro.

Thus a problem arises you need to change something.

My case was that I wanted to use ZFS and it is not included in the official Linux kernel. It is rather build as a module. On standard Debian it is done easily by installing the zfs-dkms package.

It is possible to build the module for the C1 instance kernel by preparing the build env like described here:

• https://github.com/scaleway/kernel-tools#how-to-build-a-custom-kernel-module

The problem was, that ZFS on 32bit Linux:

• "May encounter stability problems"
• "May bump up against the virtual memory limit"

which is officially stated here:

• https://github.com/zfsonlinux/zfs/wiki/FAQ#32-bit-vs-64-bit-systems

I'm stil about to see the former but hit the latter quite fast, and as recommended I had to add the vmalloc=512M boot parameter.

Unfortunately Scaleway does not support passing parameters to their kernels. They however support KEXEC via the KEXEC_KERNEL and KEXEC_INITRD params as documented here:

• https://github.com/scaleway/initrd/blob/master/Linux/README.md#boot-options

and they support parameters to the KEXEC-ed kernel via the KEXEC_APPEND param.

So as I just needed to boot the same kernel and pass the parameter. So first I had to find where the current kernel and initrd are. This is done by installing "scaleway-cli":

• https://github.com/scaleway/scaleway-cli

I've just grabbed the pre-built amd64 deb packages, and then used the "scw" command to get info about the instance:

# list servers
$ scw ps 

# Show instance details 
$ scw inspect SERVER_ID

"bootscript": {
    "bootcmdargs": "LINUX_COMMON scaleway boot=local nbd.max_part=16",
    "initrd": "initrd/uInitrd-Linux-armv7l-v3.14.6",
    "kernel": "kernel/armv7l-mainline-lts-4.9-4.9.93-rev1",
    "dtb": "dtb/c1-armv7l-mainline-lts-4.9-4.9.93-rev1",
    ...

If you inspect a VM instance you will see that the kernel and initrd are referred by IP:

"bootscript": {
    "bootcmdargs": "LINUX_COMMON scaleway boot=local nbd.max_part=16",
    "initrd": "http://169.254.42.24/initrd/initrd-Linux-x86_64-v3.14.6.gz",
    "kernel": "http://169.254.42.24/kernel/x86_64-mainline-lts-4.4-4.4.127-rev1/vmlinuz-4.4.127"

And a google search showed me that the kernel and the initrd were available at:

• http://mirror.scaleway.com/kernel/
• http://mirror.scaleway.com/initrd/

I've had a problem by trying to use the image referred in the params above:

# DO NOT USE THIS ONE
KEXEC_INITRD=http://mirror.scaleway.com/initrd/uInitrd-Linux-armv7l-v3.14.6

and I've wasted a couple of hours until I realized that this image was in a different format, not usable for the KEXEC_INITRD . Then I've changed it to:

 KEXEC_INITRD=http://mirror.scaleway.com/initrd/initrd-Linux-armv7l-v3.14.6.gz

and this time it worked fine.

The kernel can be found via at least two different URLs:

KEXEC_KERNEL=http://mirror.scaleway.com/kernel/armv7l-mainline-lts-4.9-4.9.93-rev1/vmlinuz
             http://mirror.scaleway.com/kernel/armv7l/4.9.93-mainline-rev1/vmlinuz

And after the successfull boot I've just had to add:

KEXEC_APPEND=vmalloc=512M

And my ZFS module was no longer complaining about lack of virtual memory.

Let me add a few articles that were helpful:

• Running latest kernels on ARM Scaleway servers
• https://nixos.wiki/wiki/NixOS_on_ARM/Scaleway_C1
• How to boot a kernel with different cmdline? #378
• Linux kernel in ScaleWay community

I've wasted about a day while investigating this stuff. If you find it helpful and you think that I might have saved you a couple of hours you can decide to send me a small donation on this PayPal e-mail: krustev-paypal@krustev.net