How to avoid the need for initramfs

kiigass · n00b Joined: 21 Nov 2018 Posts: 65

Hi,

I had an initramfs in my previous precious gentoo until... well Windows destroyed it =((
Now I am going to reinstall gentoo and I was wondering about the initramfs. As far as I understood what I read I am better off without initramfs, because the boot process is quicker and there is the risk to make a mistake, when updating kernel+initramfs, which results in problems on next reboot.

Different sources tell me different things, though: Most agree that, if I put /usr/ on a separate partition, I will need an initramfs. But what about /var/? Do I need an initramfs, if I put that on a separate partition? Are there other points I need to take care of?

By the way: I am going to make a Raid1 with LVM on top of it. So my questions of different partitions are rather questions of different logical volumes.

PS: Do I get into trouble (i.e. need of initramfs) because I use Raid/LVM?

thanks

NeddySeagoon · Posted: Tue Oct 13, 2020 8:40 pm Post subject:

kiigass,

Provided you can build everything into the kernel to mount root and usr is on root, no initrd is required.
With a separate /usr, you either mount it in an initrd.

With raid, its not quite so clear cut. With mdadm metadata=0.90, the kernel can do raid autoassembly.
With mdadm metadata=1.2, the default, the raid needs to be assembled in an initrd as mdadm is a user space tool.

For root in LVM, you need to do vgchange -ay in the initrd as again LVM is userspace.

You may be able to use localmount to mount /usr but to do that udev cannot be allowed to start before localmount.
here may be other things now too. Several upstreams have broken /usr on its own partition now and I've not kept up.
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charles17 · Advocate Joined: 02 Mar 2008 Posts: 3664

NeddySeagoon · Posted: Wed Oct 14, 2020 1:42 pm Post subject:

charles17,

Yep, also the kernel command line.

Build on what you know works. Start with the three separate pieces.
Once it all works, put the bits together.

If you must have an initrd, I must as I have root inside LVM on top of raid5, its possible to make it kernel agnostic by avoiding putting any kernel modules into the the initrd.

I made mine in April 2009. I've probably forgotten how to do it by now :)
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NeddySeagoon

Computer users fall into two groups:-
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kiigass · n00b Joined: 21 Nov 2018 Posts: 65

Thanks for the answers!

Ok, that means, if I want to avoid the need for initramfs/initrd, I need to:

make sure /usr/ is on the same partition as /
stick to metadata=0.90 if I use raid
go without lvm (at least on the system directories)

Is that correct?

szatox · Advocate Joined: 27 Aug 2013 Posts: 3131

NeddySeagoon · Posted: Wed Oct 14, 2020 5:29 pm Post subject:

kiigass,

The kernel must be able to mount the root filesystem without needing any user space tools.
There needs to be enough on the root filesystem to be able to boot, that means bits of /usr.
Bits of /usr in practice means all of it.
So no LVM for root.

stick to metadata=0.90 if I use raid. Yes and set raid autodetect in the kernel.

The only silly question is the one you don't ask but be on your guard for silly answers :)

If your initrd contained kernel modules that needed to be loaded to mount the root filesystem, then it is bound to the kernel its compiled for.
... but you won't have an initrd :)
Provided its only user space tools, its kernel agnostic.

My initrd contains

GDH-gentoo

NeddySeagoon · Posted: Wed Oct 14, 2020 8:33 pm Post subject:

Team,

For completness, you can have raid without an initrd or you can have root on LVM without an initrd.
For LVM on raid without an initrd, the kernel would need to start raid before it went looking for the LVM.
It does it in the wrong order.

I wrote up my findings on the wiki.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Goverp · Veteran Joined: 07 Mar 2007 Posts: 1998

FWIW: if you build your initramfs into the kernel, you'll need a script that's run every time you make a new kernel. Hence your initramfs is as current as your kernel. In my case, I use ~amd64 gentoo-sources, which means I seem to have a new kernel every week, so my initramfs is never more than a week out of date.
_________________
Greybeard

Hu · Moderator Joined: 06 Mar 2007 Posts: 21606

If you configure the kernel to embed the initramfs, the kernel build system will take care of collecting the files. There is no script to write, unless you want a custom verifier (such as checking that your list of files to include provides all supporting libraries for the programs you embedded).

kiigass · n00b Joined: 21 Nov 2018 Posts: 65

@Team: Thanks alot! I learned so much, just by reading your posts. My impression now is: I might get better performance without initramfs/initrd, but it's a bad idea, because so many things might go wrong, that my system won't boot at all some day.

pietinger · Posted: Thu Oct 15, 2020 8:44 pm Post subject:

First of all: I hate initramfs. So, I am not objectiv ...

NeddySeagoon · Posted: Thu Oct 15, 2020 10:15 pm Post subject:

kiigass,

Thats a typo in my comment. Its LVM on top of raid.

Lets get an understanding of what an initrd is. Its a root filesystem that holds all the things needed to mount the real root filesystem.
In days of old it was actually an ext2 filesystem in a file, then compressed.
You can still do that. Make an empty file, make a filesystem in the file. loop mount the file, copy things to it ...
Whatever filesystem type you choose for the initrd must be built into the kernel, or the kernel won't be able to read the filesystem.

What we do today in properly called an initramfs. Its a compressed CPIO archive of other CPIO archives.

To keep the initrd kernel agnostic, it must only contain user space tools. Never any kernel code.
If you need to load CPU microcode, you can build that into the kernel, it need not go in the initrd.

This wiki page tells how to make an initrd without the use of any autoblackmagic tools.

It can be improved. It uses binaries out of the live filesystem. That's bad as the live filesystem can change and depend on new libs.
e.g. lvm.static no longer builds. It should force the use of binaries in /root/initrd/fs, say, so that repeated runs years apart generate the same binary.
That requires the user to copy the structure to /root/initrd/fs, just like the orignal fs in a file.
plus ça change, plus c'est la même chose, as the French say. :)
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

dmpogo · Advocate Joined: 02 Sep 2004 Posts: 3267 Location: Canada

IMHO. the easiest way to avoid initrd is to have / with /usr on a separate drive, nowadays preferably SSD. And then you can play with raids, llvm etc for /var /home /usr/local /opt whatever.

Goverp · Veteran Joined: 07 Mar 2007 Posts: 1998

NeddySeagoon · Posted: Fri Oct 16, 2020 10:18 am Post subject:

Goverp,

Hu · Moderator Joined: 06 Mar 2007 Posts: 21606

msst · Apprentice Joined: 07 Jun 2011 Posts: 259

By the way, you can get most of the benefits of LVM also by simply using btrfs.

But the limitations are the same: Single disk btrfs can be booted directly by the kernel. Multi disk aka raid (which btrfs supports natively) requires a device scan and therefore an initramfs.
Note that RAID5/6 is still not considered stable, but is nowadays anyway almost obsolete.

I use on one computer an kernel with bulid-in initramfs (better-initramfs). This initramfs file is once build and then static, e.g. requires no rebuilding during kernel updates if you compile sufficient stuff in. I use it for luks-multidisk-btrfs. I also use a conventional initramfs on another computer. This requires one new script run of the initramfs per kernel update then.

Goverp · Veteran Joined: 07 Mar 2007 Posts: 1998

NeddySeagoon · Posted: Sat Oct 17, 2020 10:13 am Post subject:

Goverp,

Grub has to make its own arrangements for reading the boot files from the HDD. Grub2 can indeed pick them out from raid, LVM etc.
This is just grub getting the kernel and initrd loaded though.

Once the kernel is loaded its all on its own if there is no initrd. It has to be able to mount root itself.
Only the initrd can help. Grub is gone.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.