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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?
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The initrd image contains the necessary executables and system files to support the second-stage boot of a Linux system.
initrd provides the capability to load a RAM disk by the boot loader. This RAM disk can then be mounted as the root file system and programs can be run from it. Afterwards, a new root file system can be mounted from a different device.
In sum, ramfs is just file opened and loaded into memory, isn't it? Both initrd and ramfs are zipped at compile time, but the difference is, initrd is a block device unpacked to be mounted by the kernel at booting, while ramfs is unpacked via cpio into memory.
Creating initrdinitrd can be created with “mkinitrd” command. The location of initrd is /boot directory. The kernel version for which the initrd image is being created needs to be passed as an argument to the mkinitrd command. The current kernel version can be checked with uname command.
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.
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