The difference between initrd and initramfs?

Question

As far as I know, initrd acts as a block device, thus requiring a filesystem driver (such as ext2). The kernel must have at least one built-in module for detecting filesystem of initrd. In this article, Introducing initramfs, a new model for initial RAM disks, it is written that:

But ramdisks actually waste even more memory due to caching. Linux is designed to cache all files and directory entries read from or written to block devices, so Linux copies data to and from the ramdisk into the "page cache" (for file data), and the "dentry cache" (for directory entries). The downside of the ramdisk pretending to be a block device is it gets treated like a block device.

What's page cache and dentry cache? In the paragraph, does it mean the data got duplicated because ramdisk is treated as a block device, thus all the data is cached?

In constrast, ramfs:

A few years ago, Linus Torvalds had a neat idea: what if Linux's cache could be mounted like a filesystem? Just keep the files in cache and never get rid of them until they're deleted or the system reboots? Linus wrote a tiny wrapper around the cache called "ramfs", and other kernel developers created an improved version called "tmpfs" (which can write the data to swap space, and limit the size of a given mount point so it fills up before consuming all available memory). Initramfs is an instance of tmpfs.

These ram based filesystems automatically grow or shrink to fit the size of the data they contain. Adding files to a ramfs (or extending existing files) automatically allocates more memory, and deleting or truncating files frees that memory. There's no duplication between block device and cache, because there's no block device. The copy in the cache is the only copy of the data. Best of all, this isn't new code but a new application for the existing Linux caching code, which means it adds almost no size, is very simple, and is based on extremely well tested infrastructure.

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. Am I correct? Or is ramfs a very minimal file system?

Finally, up until this day, the initrd image is still presented in the latest kernel. However, is that initrd actually the ramfs used today and the name is just for historical purpose?

Answer 1

I think you are right in all.

The difference is easy to see if you follow the steps needed when booting:

initrd

• A ramdev block device is created. It is a ram-based block device, that is a simulated hard disk that uses memory instead of physical disks.
• The initrd file is read and unzipped into the device, as if you did zcat initrd | dd of=/dev/ram0 or something similar.
• The initrd contains an image of a filesystem, so now you can mount the filesystem as usual: mount /dev/ram0 /root. Naturally, filesystems need a driver, so if you use ext2, the ext2 driver has to be compiled in-kernel.
• Done!

initramfs

• A tmpfs is mounted: mount -t tmpfs nodev /root. The tmpfs doesn't need a driver, it is always on-kernel. No device needed, no additional drivers.
• The initramfs is uncompressed directly into this new filesystem: zcat initramfs | cpio -i, or similar.
• Done!

And yes, it is still called initrd in many places although it is a initramfs, particularly in boot loaders, as for them it is just a BLOB. The difference is made by the OS when it boots.