Why is the root filesystem is loaded into a ramdisk?

问题

I am studying the boot process in Linux. I came across this sentence "RAM is several orders of magnitude faster than a floppy disk, so system operation is fast from a ramdisk"

The kernel will anyway load the root filesystem in RAM for executing it. So my question why do we need a ramdisk for loading the the root filesystem, if the kernel loads the root file system into RAM ?

回答1:

The documentation for SUSE Linux provides a good explanation of why Linux is booted with a RAMDisk:

As soon as the Linux kernel has been booted and the root file system (/) mounted, programs can be run and further kernel modules can be integrated to provide additional functions. To mount the root file system, certain conditions must be met. The kernel needs the corresponding drivers to access the device on which the root file system is located (especially SCSI drivers). The kernel must also contain the code needed to read the file system (ext2, reiserfs, romfs, etc.). It is also conceivable that the root file system is already encrypted. In this case, a password is needed to mount the file system.

For the problem of SCSI drivers, a number of different solutions are possible. The kernel could contain all imaginable drivers, but this might be a problem because different drivers could conflict with each other. Also, the kernel would become very large because of this. Another possibility is to provide different kernels, each one containing just one or a few SCSI drivers. This method has the problem that a large number of different kernels are required, a problem then increased by the differently optimized kernels (Athlon optimization, SMP). The idea of loading the SCSI driver as a module leads to the general problem resolved by the concept of an initial ramdisk: running user space programs even before the root file system is mounted.

This prevents a potential chicken-or-egg situation where the root file system cannot be loaded until the device on which it is located can be accessed, but that device can't be accessed until the root file system has been loaded:

The initial ramdisk (also called initdisk or initrd) solves precisely the problems described above. The Linux kernel provides an option of having a small file system loaded to a RAM disk and running programs there before the actual root file system is mounted. The loading of initrd is handled by the boot loader (GRUB, LILO, etc.). Boot loaders only need BIOS routines to load data from the boot medium. If the boot loader is able to load the kernel, it can also load the initial ramdisk. Special drivers are not required.

Of course, a RAMDisk is not strictly necessary for the boot process to take place. For example, you could compile a kernel that contained all necessary hardware drivers and modules to be loaded at startup. But apparently this is too much work for most people, and the RAMDisk proved to be a simpler, more scalable solution.

回答2:

The reason that most Linux distributions use a ramfs (initramfs) when booting, is because its contents can be included in the kernel file, or provided by the bootloader. They are therefore available immediately at boot, without the kernel having to load them from somewhere.

That allows the kernel to run userspace programs that e.g. configure devices, load modules, setup that nifty RAID array that contains all filesystems or even ask the user for the password to his encrypted root filesystem.

When this configuration is done, the first script that is called just exec()s /sbin/init from the (now configured and available) root filesystem.

I have seen quite a few systems where the drivers themselvess for the disk controllers and the rootfs are loaded via modules in an initramfs, rather than being included in the kernel image.

You do not strictly need an initramfs to boot - if your kernel image contains all drivers necessary to access the rootfs and you don't need any special configuration or user input (like RAID arrays or encrypted filesystems) to mount it, it is often possible to directly start /sbin/init from the rootfs.