does rescue kernel ever update

[Jon LaBadie]

On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.

Are old rescue kernels still useful? (6 years?)

Are there automated or manual procedures to update
a rescue kernel?

Are there best practices for rescue kernel update?
If there are, I've missed them.

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[Matthew Miller]

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.

Post by Jon LaBadie
Are old rescue kernels still useful? (6 years?)

Yes -- they will let you boot into the system. The rescue initrd includes
all available drivers and so can boot even if the drive is in a very
different system from the one it was installed on.

Post by Jon LaBadie
Are there automated or manual procedures to update
a rescue kernel?

There's generally no reason to. But you can with
/etc/kernel/postinst.d/51-dracut-rescue-postinst.sh $(uname -r) /boot/vmlinuz-$(uname -r)

Post by Jon LaBadie
Are there best practices for rescue kernel update?
If there are, I've missed them.

That's because the best practice is generally to not worry about it.

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[Lester M Petrie]

Post by Matthew Miller

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.

Post by Jon LaBadie
Are old rescue kernels still useful? (6 years?)

Yes -- they will let you boot into the system. The rescue initrd includes
all available drivers and so can boot even if the drive is in a very
different system from the one it was installed on.

Post by Jon LaBadie
Are there automated or manual procedures to update
a rescue kernel?

There's generally no reason to. But you can with
/etc/kernel/postinst.d/51-dracut-rescue-postinst.sh $(uname -r) /boot/vmlinuz-$(uname -r)

I am on F33, but I don't have such a file. What creates it, or where
does it come from?

Post by Matthew Miller

Post by Jon LaBadie
Are there best practices for rescue kernel update?
If there are, I've missed them.

That's because the best practice is generally to not worry about it.

--
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[Joe Zeff]

Are old rescue kernels still useful?  (6 years?)

They're still just as useful as they were when they were installed. Of
course, that means that any function that was added later isn't there,
but that doesn't matter because you're only going to use it in
emergencies to troubleshoot a broken system.
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[Patrick O'Callaghan]

Are old rescue kernels still useful?  (6 years?)

They're still just as useful as they were when they were installed. 
Of
course, that means that any function that was added later isn't there,
but that doesn't matter because you're only going to use it in
emergencies to troubleshoot a broken system.

Surely an old rescue kernel may not be able to mount a BTRFS
filesystem?

poc
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[Joe Zeff]

Post by Patrick O'Callaghan
Surely an old rescue kernel may not be able to mount a BTRFS
filesystem?

BTRFS has been a fully stable part of the kernel since 2013. How old is
your rescue kernel?
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[Patrick O'Callaghan]

Post by Patrick O'Callaghan
Surely an old rescue kernel may not be able to mount a BTRFS
filesystem?

BTRFS has been a fully stable part of the kernel since 2013.  How old
is
your rescue kernel?

It was a theoretical question. My rescue kernel is recent.

poc
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[Joe Zeff]

Post by Patrick O'Callaghan
It was a theoretical question. My rescue kernel is recent.

As I expected. I answered the way I did as a form of reducto ad absurdum.
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[Jon LaBadie]

Post by Joe Zeff

Post by Patrick O'Callaghan
Surely an old rescue kernel may not be able to mount a BTRFS
filesystem?

BTRFS has been a fully stable part of the kernel since 2013. How old
is your rescue kernel?

Close, one system's rescue kernel is early 2015, but others
could have even older rescue kernels.

But I believe POC's point was not about BTRFS specifically,
but that something could have changed over the years to make
an old rescue kernel unusable.

If you accept that premise, shouldn't the rescue kernel be
periodically updated?

--
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[George N. White III]

Post by Jon LaBadie
But I believe POC's point was not about BTRFS specifically,
but that something could have changed over the years to make
an old rescue kernel unusable.
If you accept that premise, shouldn't the rescue kernel be
periodically updated?
Drivers come and go and come back, so older hardware may

not be supported in newer kernels. The rescue kernel should
work as well as it did for the original installation until some
major hardware change like a new wifi card.

I have a couple old (>10 years) systems. For one, wifi was
broken for many years (so I used a USB dongle) but does
work in recent kernels. For the other, the ethernet driver was
dropped by the kernel for a year or two (and has now come
back).

The worst case would be a hardware failure that requires
adding a new device (e.g., USB wifi or ethernet when the
original device dies) that isn't supported by the old rescue
kernel.

[Chris Murphy]

On Thu, Jun 3, 2021 at 3:19 PM Patrick O'Callaghan

Post by Patrick O'Callaghan

Post by Joe Zeff

Post by Jon LaBadie
Are old rescue kernels still useful? (6 years?)

They're still just as useful as they were when they were installed.
Of
course, that means that any function that was added later isn't there,
but that doesn't matter because you're only going to use it in
emergencies to troubleshoot a broken system.

Surely an old rescue kernel may not be able to mount a BTRFS
filesystem?

Not only Btrfs but any file system. A new mkfs may set options that an
old kernel doesn't support. There's quite a bit of that in ext4 and
XFS land.

If you mkfs.btrfs with today's progs and use defaults, an ancient
kernel will mount it. But there are features that old kernels don't
support, like for example zstd compression which arrived in kernel
4.14, thus not mountable (incompatible) with older kernels. Free space
tree v2 exists since kernel 4.5, but its flag permits ro mount with
old kernels.

--
Chris Murphy
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[Samuel Sieb]

Post by Jon LaBadie
Are there automated or manual procedures to update
a rescue kernel?

If you delete the rescue kernel and initrd, they will be recreated the
next time you install a kernel.
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[old sixpack13]

Post by Samuel Sieb
If you delete the rescue kernel and initrd, they will be recreated the
next time you install a kernel.

[KlugscheissMode on]
what is only valide when:
- no one has changed the default "yes" to "no" in /usr/lib/dracut/dracut.conf.d/02-rescue.conf
- a new dracut was installed/updated after the above change to "no", cause dracut obvious doesn't *respect* user settings, Hrmph, :-(
[KlugscheissMode off]
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[Chris Murphy]

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.
Are old rescue kernels still useful? (6 years?)

They might be useful to a sysadmin, I think they are useless. The
rescue kernel is really just a "no host-only" initramfs that contains
a bunch of extra dracut and kernel modules that the host only
initramfs doesn't. The difficulty is the rescue initramfs can't do a
full graphical boot once /usr/lib/modules/ dir for that kernel has
been removed, which was likely in its first 4 weeks following
installation.

Since you won't get graphical boot anyway, I'm not sure you have a
good chance of dracut building a new host only initramfs that
contains the driver needed for whatever new hardware you've added or
changed to. It's pretty esoteric landing in a dracut shell even for
experienced users. So I am not a fan.

What I would like to see is (a) an initramfs that can boot a graphical
stack (b) contains the Live OS dracut modules (c) and overlayfs, and
wire it up so that the rescue boot entry does a read-only sysroot boot
+ writable overlay like a LiveOS. So now folks can use a web browser
normally, get on irc or whatever, and get some help with why they
can't boot without having to resort to mobile or a 2nd computer they
may not have handy.

A side plus for Btrfs cases, it has a unique ro,rescue=all mount
option that tolerates file system problems. Plausibly we can still
boot read-only in situations where other file systems would face plant
until they get an fsck. Whereas on Btrfs we really want to steer folks
towards freshening backups before they attempt a repair, if they end
up in a disaster situation. But nevertheless, such an effort would be
generically beneficial no matter the file system.

A variation on that might be a read-only "rescue" or "recovery"
snapshot that would be immutable, paired with the same
LiveOS+overlayfs concept. That way a boot is possible in a variety of
other more likely user error or update related scenarios, i.e. file
system isn't damaged, it's the installation that's messed up, and what
you need is a live boot but don't have a USB stick handy, so just bake
it into a small snapshot. *shrug* maybe. We could make it completely
self contained including the kernel, initramfs, and the whole graphics
stack. But, near term such a thing would be btrfs only unless we
dedicate a literal partition and stick a Live OS ISO image on it
(effectively).

--
Chris Murphy
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[Tim via users]

Post by Chris Murphy
What I would like to see is (a) an initramfs that can boot a
graphical stack (b) contains the Live OS dracut modules (c) and
overlayfs, and wire it up so that the rescue boot entry does a read-
only sysroot boot + writable overlay like a LiveOS. So now folks can
use a web browser normally, get on irc or whatever, and get some help
with why they can't boot without having to resort to mobile or a 2nd
computer they may not have handy.

I really don't know how anybody manages to fix a middling computer
problem without having a second computer. You nearly always need to
research something, or yank a drive and copy files over in one
direction or the other. Trying to do that on a banjaxed computer is
really difficult.

--

uname -rsvp
Linux 3.10.0-1160.25.1.el7.x86_64 #1 SMP Wed Apr 28 21:49:45 UTC 2021 x86_64

Boilerplate: All unexpected mail to my mailbox is automatically deleted.
I will only get to see the messages that are posted to the mailing list.

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[Patrick O'Callaghan]

Post by Tim via users

Post by Chris Murphy
What I would like to see is (a) an initramfs that can boot a
graphical stack (b) contains the Live OS dracut modules (c) and
overlayfs, and wire it up so that the rescue boot entry does a read-
only sysroot boot + writable overlay like a LiveOS. So now folks can
use a web browser normally, get on irc or whatever, and get some help
with why they can't boot without having to resort to mobile or a 2nd
computer they may not have handy.

I really don't know how anybody manages to fix a middling computer
problem without having a second computer.  You nearly always need to
research something, or yank a drive and copy files over in one
direction or the other.  Trying to do that on a banjaxed computer is
really difficult.

Yep. You can sometimes make progress with a Live Image but I keep an
ancient laptop (circa 2008) around just for this.

poc
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[Jon LaBadie]

Post by Chris Murphy

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.
Are old rescue kernels still useful? (6 years?)

They might be useful to a sysadmin, I think they are useless. The
rescue kernel is really just a "no host-only" initramfs that contains
a bunch of extra dracut and kernel modules that the host only
initramfs doesn't. The difficulty is the rescue initramfs can't do a
full graphical boot once /usr/lib/modules/ dir for that kernel has
been removed, which was likely in its first 4 weeks following
installation.
Since you won't get graphical boot anyway, I'm not sure you have a
good chance of dracut building a new host only initramfs that
contains the driver needed for whatever new hardware you've added or
changed to. It's pretty esoteric landing in a dracut shell even for
experienced users. So I am not a fan.

Might it be useful for /usr/lib/modules/rescue... (and maybe also
/usr/src/kernels/rescue...) to be created/preserved for the rescue
kernel?

--
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[Doug H.]

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.
Are old rescue kernels still useful? (6 years?)
Are there automated or manual procedures to update
a rescue kernel?
Are there best practices for rescue kernel update?
If there are, I've missed them.

I don't think anybody gave an example of procedure to update...

1. Turn off auto updates to dnf.
2. Daily update: sudo dnf upgrade
2a. If there is not kernel update hit "Yes" and stop here.
2b. If there is a kernel update hit "No"
3. Delete the "rescue" stuff in /boot and /boot/loader/entries/
4. sudo dnf upgrade -y
5. Reenable auto updates if you wish.

This will make sure you get your rescue built right away without having to push the build yourself. This just lets the system do it for you. You could just do the deletes and then wait, but it might be days before a new kernel comes in to push the rebuild.


--
Doug Herr
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[Jon LaBadie]

Post by Doug H.

Post by Jon LaBadie
On my 3 systems, F34, F34, and CentOS7, they are
1, 2, and 6 years old respectively.
Are old rescue kernels still useful? (6 years?)
Are there automated or manual procedures to update
a rescue kernel?
Are there best practices for rescue kernel update?
If there are, I've missed them.

I don't think anybody gave an example of procedure to update...
1. Turn off auto updates to dnf.
2. Daily update: sudo dnf upgrade
2a. If there is not kernel update hit "Yes" and stop here.
2b. If there is a kernel update hit "No"
3. Delete the "rescue" stuff in /boot and /boot/loader/entries/
4. sudo dnf upgrade -y
5. Reenable auto updates if you wish.
This will make sure you get your rescue built right away without having to push the build yourself. This just lets the system do it for you. You could just do the deletes and then wait, but it might be days before a new kernel comes in to push the rebuild.

Thanks for the stepwise instructions Doug.

One thought, should one be certain that they are happy with
the most recent kernel before embarking on replacing the
rescue kernel. I don't reboot every time a kernel is installed.
So I may not have yet run the most recent. Murphy's law says
that will be the one that breaks something and I wouldn't want
it as my rescue kernel ;)

Slightly off topic, my really old rescue kernel (2015) is on a
CentOS system and has no /boot/loader directory. Was there an
alternative before /boot/loader was introduced?

--
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[Sam Varshavchik]

Post by Doug H.
3. Delete the "rescue" stuff in /boot and /boot/loader/entries/

We should have a canned wrapper for this. We already have

dnf config-manager --set-enabled repo

to simply flip a single character in a .conf file.

It will be helpful in avoiding typos to have something similar, for removing
the rescue kernel.