Currently I am learning about linux booting process.Here i noticed that the initrd will create temporary root file system that includes drivers(LVM, NFS, etc) which are required by kernel.After that kernel will make use of that drivers and it will mount real root file system.
Here my question is why not kernel itself should include the necessary drivers inside it and why it depends on initrd ?
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Firstly, know that kernel memory is not demand-paged. That would be a circular dependency. If you page out your disk driver to the disk when you're low on memory, you can't load it back later.
(And inside the Linux kernel, we don't try to define some higher layer which isn't involved in the storage path and can safely be paged out. Allegedly this is possible: Windows did it. I don't know what this higher layer is supposed to be though. Maybe it is defined dynamically. Or maybe Windows has the luxury of not supporting strange ideas like swap-over-NFS).
Instead, we support loading modules. If we don't need NFS on this particular computer, we don't have to load it.
In a modern distribution this saves on the order of a hundred megabytes of RAM overall. (Look at the space taken by /lib/modules/$VERSION/. Note that in modern distributions, the modules are compressed e.g. .xz files).
While kernel modules are the more obvious reason for the initrd, there is a second aspect.
It lets userspace build arbitrarily complex storage stacks to access the root filesystem. E.g. getting an IP address using DHCP, to support NFS, or prompting for a disk encryption passphrase.
Again, the kernel tries not to bloat too much e.g. with user interface code.
Memory usage is only one reason. The kernel/userspace division is overloaded with a number of purposes. E.g. the kernel can be one project and work specifically on kernel things. Userspace can be anything; it could be a "normal" Linux distribution, or it could be an entirely independent project like the Android OS.
This is different from other OS's, e.g. the BSDs maintain kernel + core userspace together. This is illustrated by BSDs being able to handle the 2038 problem with a single flag day conversion of both kernel and userspace.
It doesn't if you want to embed the drivers within the kernel. A great source based linux distribution that can help with using a custom kernel configuration would be Gentoo or you can simply use the current distributions package manager to acquire the Linux Kernel Source (usually a dev and headers package) and use the make config to step through the required hardware required for the target system and change each value from M to *.
More detailed instructions can be found in the documentation
