This might be a stupid question, but I think I have missed some information somewhere.

In /etc/fstab are the partitions to be mounted
/ is one of those partitions
/etc/fstab is a subfolder of /

How can /etc/fstab be read, to know that / has to be mounted before it is mounted?

When you start up, you load a ram based system (initrd) first, before transferring to the disk based system.

initrd = initial ramdisk

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Then I have to read up on that.
Thanks.

But what is the function of the root partition line in fstab given that the system already knows where its root is? Is that line just for the benefit of the sysadmin or does it actually do something?

Good question. I assume that you can unmount / for whatever reason and then mount it again with mount -a

No, you can't umount the root. But you can remount it rw in need - which needs an entry in fstab IIRC.

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I never thought remount was different from "unmount then mount", but I guess in case of / it makes a big difference.

Also explains how /bin/mount doesn't need to be used, once / would have been dismounted... it all makes sense.

Now I get it. Root is usually mounted read only to start with in case it needs to be fscked. Then it gets remounted read-write. Having it listed in fstab makes that sequence unproblematic.

And, actually, it is possible to avoid the "initrd" step entirely.

The initrd (or initfs) file is a ramdisk image of the actual root file system. It needs all the parts (including everything in /etc) to be there when the initrd file is created so that the initial load of the file system from ramdisk has everything that is needed to load the kernel and start the system up properly. Once the kernel is loaded and the system is running then the actual root file system is now mounted over the ramdisk image and that memory is freed up for other use. The system only knows where the actual root file system is by reading the content of /etc/fstab in the initrd image file.

There are 4 basic files that are needed to handle booting on my system (and all exist in the /boot directory).
config, initrd (initramfs on fedora), vmlinuz (the actual kernel), and System.map. All are needed for each kernel version installed and bootable.

That's not the case on my system. I use elilo and it passes a kernel command line of the form "append="root=/dev/sda7 vga=normal ro". Lilo used to do the same. I don't know much about GRUB.

I said that wrong. The system needs certain files and the actual file system structure that is contained in the initrd once the kernel is loaded. The kernel itself has to have that structure available before it can actually finish setting up the system then mount the drives, which happens quite a ways into the boot process.

My system uses systemd and journalctl for logging and I see more than 700 lines in the logs before it even sees and starts to configure the sata devices, then another 150 lines in the log before it configures the boot drive.

This means the initrd image has been in use for some time before the system is able to use the main root file system.

I believe that the initrd system looks for & reads the /etc/fstab to find the location of the system to be transferred into by 'switchroot' or some other mechanism.

root= in bootloader is optional in most installations. root= is normally included in initrd, making the one in bootloader superfluous, unless the location in the initrd isn't valid for the current boot, such as a clone migration where the clone / filesystem has different UUID, LABEL and/or device name. root= in bootloader is also required when there is no initrd.

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