As I understand, initrd is a small image that is loadable in the RAM. It is used to boot a complete kernel with all the loadable modules. As part of the process, we need the vmlinuz kernel image which is a renamed version of bzImage.
Is it possible to boot the kernel without creating the initrd image?
initrd/initramfs is optional and not a requirement. bzImage is the pure kernel image and can be booted directly by the bootloader. However it might be neccesary to execute some tasks (loading filesystem modules, drivers for disk access, mounting the root file system from some exchangeable media without fixed name/path, etc.) that would usually require access to a filesystem and userspace tools.
That's what initramfs is for: It is a CPIO archive that gets attached to the kernel image (the kernel image is the container for the initramfs not other way round) either in the kernel image itself, or by the bootloader at boot time.
That CPIO archive contains an initial rootfs with the modules required to setup all devices to access the proper root filesystem and some programs to identify those devices, load the modules, do some other startup tasks remount the proper root file system to / and start /sbin/init
initrd is similar, with the main difference that it is an filesystem image, that may be and usually is compressed. The kernel must have support for the filesystem used built in and will mount this image as the initial /.
Since CPIO is simpler by several orders of magnitudes, initramfs is prefered over initrd, as this saves both the requirement for any filesystem modules being built in and also makes initramfs creation easier. Instead of having to create an ext2 image, loopdevice mount and populate it, it boils down to a simple archive creation, not unlike using tar.
However if you compile your kernel with all required drivers and modules built into the kernel image, and your root file system device has a fixed name in the system you don't need a initramfs as the kernel can do things by itself then.
The initrd contain the modules required to understand the root filesystem, and thus be able to access the normal store of kernel modules.
If your kernel is compiled with all that code built-in, rather than as modules, then an initrd shouldn't be required.
Minimal QEMU + Buildroot example
Here is a minimal concrete example that shows that initrd is not mandatory: https://github.com/cirosantilli/linux-kernel-module-cheat/tree/0b4f156b1b536a89c90882ed8ce551abcd3780af#initrd
With that setup, we can easily run two working QEMU commands of type:
qemu-system-x86_64 -drive file=rootfs.ext2
and:
qemu-system-x86_64 -initrd rootfs.cpio
Where:
rootfs.ext2 and rootfs.cpio are basically the same root filesystem, but in different formats-initrd-initrd but no hard driveIn both cases, Linux boots fine, except that in the -initrd system, file writes are not persistent since everything is in memory.
Yes, you can boot a system without an initrd image.
initrd image is either a gzipped ramdisc image, or (more commonly nowadays) a gzipped .cpio image.
In the latter case, the .cpio is expanded into a filesystem called initramfs.
If the .cpio image isn't present, the kernel uses a built-in image instead, which contains just a few special files (such as /dev/console, /dev/null and a few directories), but no binaries.
The kernel then uses some built-in logic and command-line options to try to find and mount is "real" root filesystem, which is mounted "over" the initramfs and therefore hides it.
This "legacy" boot system is mostly not used in modern distributions.
