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- Now that we know how grub is organized,

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let's do some grub troubleshooting.

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Let me create some trouble.

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(keyboard clacking)

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This command will override

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the boot checker, the mass boot record,

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the part that contains
the grub configuration.

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Look, if you do an xxd -l 512

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on dev/sda, there's nothing to boot from.

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Notice that I did not
wipe the partition table.

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That would bring me a completely
different range of problems

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Now I'm going to reboot,

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and first we are going
to see what's happening.

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So, this is what's happening

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if you don't have a massive
boot record anymore.

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The system cannot boot
from your hard drive

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and the power on self disk will look

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for the next bootable device.

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And the bootable device, in this case,

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is the installation disk.

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Which is nice because this
is the only thing that I can

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use to get out of this situation.

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We need to boot a rescue system.

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So, I'm selecting a
rescue sent to a system.

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And we'll start booting from there.

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We need to give it a minute

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to load the entire rescue environment.

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So, we need to select step one,

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which will look for the
partition or logical volume

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containing the root file system.

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And yes, we want to
mount it on mnt/sysimage

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so let's press RETURN,

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the old way to press ENTER.

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And now we can use chroot/mnt/sysimage

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We need to do that anyway.

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Now we need to type mount to
figure out what is mounted.

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And very nice, we can se
that /devmapper/centos-root

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is mounted on xfs, it's
a root file system.

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We can also see that
/dev/sda1 is mounted on /boot

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which is a requirement, as well,

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because that is the boot partition.

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And we need to reinstall grub

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and in order to reinstall grub

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you need access to the
contents of the boot partition.

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So, how are we going to reinstall grub?

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Well, that will be one
of the grub2 utilities.

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You can see that grub2 has many utilities.

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Most of them you will never use.

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But in this case we are
going to use grub2-install

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on /dev/sda

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Obviously, you have verified
the name of your disk device

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before you do so.

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And here we can see that it has installed.

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Installation finished, no error reported.

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So, let's see if we can
do xxd -L 512 on /dev/sva

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And that looks like boot code.

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This is the good old grub boot
logger as we have seen before

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So, from here we can type exit

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and next we can reboot

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and verify that the grub
recovery was successful.

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We see a grub boot menu again.

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So, this was successful.

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Another thing that you need to know about

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is manually booting from the grub2 prompt.

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I've summarized the
procedure on the slide.

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So, let's walk through the slide

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before I will tell you how to do this

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from a live system.

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So, in the grub2 prompt,
first step is that

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you need to load the required modules.

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These modules provide
access to your storage.

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Typically that will be xfs, lvm, and ext4.

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Then you will use the ls command

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to check the current disk layout.

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And based on the current disk layout,

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you can prompt for the contents
of the /boot directory.

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For a change in this slide,
I've used ls on hd0,gpt1

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which is a gui partition number one.

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In my demo I will do it on a bios system

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so we will have hd0, MSdos1.

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Next, we will look for
the kernel, the initramfs,

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as well as the root partition.

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And we will set the root like this.

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set root is lvm/fedora at -root

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And then we can declare the kernel.

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So, the kernel is linux
which is on hd0,gpt1

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And which has the name /vmlinuz whatever

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Use Tab completion to
make that name complete.

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Following that there is root

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which specifies the root volume to,

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in this case /dev/fedora/root

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And then you continue by
loading the /initramfs

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Once you have loaded kernel
as well as /initramfs

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you can type boot.

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And if you like what you have just done,

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once you are in the
operational system again,

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you can type grub2-mkconfig
-o /boot/grub2/grub.cfg

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And then your system will
be fully functional again.

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Let me show you how to do this.

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So, if you want to manually boot grub,

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from the grub menu,
provided that you see it,

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you can type C

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In some scenarios, you
will automatically drop

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in the grub shell.

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The help command is
useful, and it's giving you

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an overview of all that is
possible from the grub shell.

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And as you can see, there's
quite a lot of commands.

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Now, what we are going to do is insmod.

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So, insmod xfs

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to start it.

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Then insmod lvm

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And well, I don't really
need it, but for completeness

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let's include it, as well:

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insmod ext4

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Oh, we don't have an ext4 module.

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Alright, let's do it without ext4.

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Depends on the system
configuration, of course.

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Next, you will use ls.

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ls is quite important because it will

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reveal the current disk layout.

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So, we can see that bios
recognizes one drive: hd0

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And another drive: hd1

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And it shows us the partitioning
that exists on the drive.

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So hd0 MSdos1

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and hd0 MSdos2.

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That's probably what we're
going to need in order to boot.

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If you are not sure which one you need,

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then just use ls to figure
out what is available.

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So, ls hd0, MSdos2/

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It's giving us unknown filesystem.

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So, that's probably not what we need.

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Or, maybe you haven't loaded
the appropriate module.

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Let's try the same on MSdos1

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and on MSdos1 we can see the contents

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of this filesystem.

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And if you look carefully,
you can recognize this

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as the contents of the boot directory.

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So, we need to look for initramfs

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and vmlinuz.

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You can see them in the list
of files listed right here.

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So, that is what we are going to access.

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But also, before going there we need to

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set the root filesystem.

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So, set root =

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set root = what?

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Well, look a couple of lines above

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in the result of the ls command.

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We need to specify the lvm logical volume.

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In grub compatible notation.

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So, lvm/centos-root

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That is going to be the root filesystem.

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And now we can specify
which kernel we want to use.

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So, the kernel is going
to be on hd0, MSdos1

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/ vmlinuz

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I'm using tab completion because
it's way more convenient.

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3. Most recent version

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is 21 and that is the kernel.

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You know that the kernel can be loaded

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with a couple of different arguments.

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There is only one argument
that is really required,

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and that is the specification
of the root filesystem.

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So that will be root=/dev/centos

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/root

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Notice that at this point
there is no tab completion

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because the kernel hasn't been loaded yet.

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And the root that we specify here

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is a kernel name.

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Press ENTER

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and if it doesn't complain

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then hopefully it will be
happy and you can continue.

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Now that the kernel has been specified

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we can specify the initramfs

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initrd, which is the
old name for initramfs,

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its initram drive.

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So, hd0, MSdos1

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/ which brings us to
the root of the contents

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of this last boot directory.

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initramfs TAB

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So, what do we need?

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We need the 3. whatever image

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subversion 21, and press ENTER
to initialize that, as well.

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And if all was successful at this point

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you can type boot.

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That looks like a booting system,

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so probably we're good.

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And now I'm sure that we are good

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because you can see that right now

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it is processing the different
system, the unit files.

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So, as we have manually
booted the system now,

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I can log in

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and finally open a root shell to make

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the new configuration persistent.

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(keyboard clacking)

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So, to make the new
configuration persistent,

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use grub2-mkconfig

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You know what happens if
you don't specify the -o?

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It writes to the standard out.

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So, it has just written
the grub configuration

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to your screen.

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That might be very interesting to see,

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but it's not useful.

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Because you cannot boot from a screen.

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In order to boot, you
need to write the output

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to the configuration file.

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Which is -o for output /boot/grub2

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/grub.cfg

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Now it has created the
grub configuration file

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and at this point you are ready to boot

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from this new configuration.

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Which brings us to the
end of this section.

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I have a lab for you, a lab
without a solution this time.

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00:10:59,850 --> 00:11:01,920
The lab is to use the grub2 prompt

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00:11:01,920 --> 00:11:04,360
to start your system manually.

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00:11:04,360 --> 00:11:05,830
If you want to verify it,

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00:11:05,830 --> 00:11:08,350
it's the procedure that
we have just discovered.

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00:11:08,350 --> 00:11:09,183
Good luck!