December 2014 - Linux Global

This writeup covers the process of configuring CentOS 7 to boot seamlessly from an ATAoE target.

This is our setup:

TFTP server with a MAC-specific boot menu presenting the kernel and initrd. See http://www.syslinux.org/wiki/index.php/PXELINUX
Server exporting an LVM Logical Volume with a CentOS 7 install on it using vblade.
Computer to boot from the network (This computer has no hard drive and is using the exported LV as it’s disk).

Required components:

A kernel that supports AoE and your network card (We are using a custom build 3.17.4 kernel. If you are building your own kernel, the driver for AoE is in Device Drivers —> Block Devices and is called ATA over Ethernet Support. The driver for your network card is in Device Drivers —> Network device support —> Ethernet driver support. The driver for your NIC will be in there somewhere under the appropriate manufacturer).
A custom dracut module to bring up the network and discover AoE targets at boot time.
Packages:
- vblade (This is only required on the server exporting the LV).
- Dracut (dracut, dracut-network, dracut-tools)

Required Steps:

1. Install CentOS 7 onto an LV on the boot server. Since this is not in the scope of this tutorial, I will leave this part up to you. There is plenty of documentation out there (we installed it under KVM first, and exported the resulting disk as an AoE target).
2. Once you have it installed, you need to enter the environment that you deployed in step 1 to install a dracut AoE module and generate an initrd.
3. Next, make sure that you have a kernel that supports aoe and your network card. It is important that both of those kernel modules get built into your initrd or this will not work.
4. At this point (hopefully) we are in the CentOS 7 environment, whether chrooted, virtual machine, or something else; we can now install packages:

1. yum install dracut-network dracut-tools dracut
2. cd /usr/lib/dracut/modules.d/ && ls

You’ll notice that there are a bunch of folders in here, all starting with two digits. The digits signify the order in which the modules are loaded. To make sure all of the prerequisite modules are loaded, we went with 95aoe for our module directory.

3. mkdir 95aoe && cd 95aoe
4. There are 3 files that we need to create — module-setup.sh, parse-aoe.sh, and aoe-up.sh

1. Let’s start with module-setup.sh, since this is the script that will pull into the initrd all of the pieces we need for AoE to work.

#!/bin/bash
# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
# ex: ts=8 sw=4 sts=4 et filetype=sh

check() {
        for i in mknod ip rm bash grep sed awk seq echo mkdir; do
                type -P $i >/dev/null || return 1
        done

        return 0
}

depends() {
        echo network
        return 0
}

installkernel() {
        instmods aoe
}

install() {
        inst_multiple mknod ip rm bash grep sed awk seq echo mkdir

        inst "$moddir/aoe-up.sh" "/sbin/aoe-up"
        inst_hook cmdline 98 "$moddir/parse-aoe.sh"
        dracut_need_initqueue
}

2. Next is parse-aoe.sh. This script will load the modules and queue up our main script to be run by init.

#!/bin/sh

modprobe aoe
udevadm settle --timeout=30
/sbin/initqueue --settled --unique /sbin/aoe-up

3. Finally, we need aoe-up.sh, which is what is actually run by init to bring up the network interfaces and discover the AoE device being exported. It’s worth noting that I am setting the MTU to 9000 on each interface, which won’t necessarily be supported on your system. if you are unsure, remove the line “ip link set dev ${INTERFACES[$i]} mtu 9000”:

#!/bin/bash

PATH=/usr/sbin:/usr/bin:/sbin:/bin

exec >>/run/initramfs/loginit.pipe 2>>/run/initramfs/loginit.pipe

mkdir -p /dev/etherd
rm -f /dev/etherd/discover
mknod /dev/etherd/discover c 152 3

INTERFACES=(`ip link |grep BROADCAST |awk -F ":" '{print $2}' |sed 's/ //g' |sed 's/\n/ /g'`)
TOTAL_INTERFACES=${#INTERFACES[@]}

for i in `seq 0 $(($TOTAL_INTERFACES-1))`; do
    ip link set dev ${INTERFACES[$i]} mtu 9000
    ip link set ${INTERFACES[$i]} up
    wait_for_if_up ${INTERFACES[$i]}
done

ip link show

echo > /dev/etherd/discover

5. Now that we have written our AoE dracut module, we need to rebuild the initrd. Before we do this however, we need to make sure that dracut will pull in the modules we need (aoe and your NIC module). There are a few different ways to do this, but here are two options:

1. modprobe aoe && modprobe NIC_MODULE && dracut –force /boot/initramfs-KERNEL_VER.img KERNEL_VER
2. dracut –force –add-drivers aoe –add-drivers NIC_MODULE /boot/initramfs-KERNEL_VER.img KERNEL_VER

6. Now that we have our new initrd, we need to transfer the kernel and initrd to our tftp server

1. scp /boot/initramfs-KERNEL_VER.img /boot/vmlinuz-KERNEL_VER TFTP_SERVER:/path/to/tftp
2. Make sure that permissions are set correctly (should be chmod 644)

7. Before we shut down our CentOS 7 VM, there is one last bit of configuration we need to adjust. Since we require the network to be active to do anything with our FS, we need to make sure that on shutdown, the network isn’t brought down before the FS is unmounted. In CentOS 6, this was as easy as running `chkconfig –level 0123456 on`. CentOS 7 uses systemd, and so this solution will not work. Fortunately the solution is simple (and probably works in CentOS 6 too): Modify /etc/fstab, adding the option _netdev to each mount point that is being exported with AoE (e.g. change defaults to defaults,_netdev).
8. Now we need to get vblade (http://sourceforge.net/projects/aoetools/files/vblade/) and put it on the system exporting the LV with CentOS 7 on it. If you don’t want to install it onto your system, you can just make it and run it from where you extracted it. To export the disk, just run:

1. vbladed -b 1024 -m MAC 0 1 INTERFACE /path/to/disk (for information on what each command does, check out the vblade man page, which is included with the package)

And that’s it! I recommend at this point to run dracut on your netbooted hardware to make sure everything still loads (at this point you shouldn’t have to specifically install the AoE module or your NIC module, so make sure that this is true). Don’t forget to copy the newly created initrd to the TFTP server, and I recommend not overwriting your working initrd so that you can easily go back to a known working state.

What took me days to get working should now only take you an hour or so! I tried to be as detailed as I could, and I don’t think I left anything out, but if you have any issues, please let me know and I’ll update this guide accordingly.

Month: December 2014

Netboot CentOS 7 on ATAoE