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?
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Answer
Dentry (and inode) cache
Filesystem subsystem in Linux has three layers. The VFS (virtual filesystem), which implements the system calls interface and handles crossing mountpoints and default permission and limits checks. Below it are the drivers for individual filesystems and those in turn interface to drivers for block devices (disks, memory cards, etc.; network interfaces are exception).
The interface between VFS and filesystem are several classes (it’s plain C, so structures containing pointers to functions and such, but it’s object-oriented interface conceptually). The main three classes are inode, which describes any object (file or directory) in a filesystem, dentry, which describes entry in a directory and file, which describes file open by a process. When mounted, the filesystem driver creates inode and dentry for it’s root and the other ones are created on demand when process wants to access a file and eventually expired. That’s a dentry and inode cache.
Yes, it does mean that for every open file and any directory down to root there has to be inode and dentry structures allocated in kernel memory representing it.
Page cache
In Linux, each memory page that contains userland data is represented by unified page structure. This might mark the page as either anonymous (might be swapped to swap space if available) or associate it with inode on some filesystem (might be written back to and re-read from the filesystem) and it can be part of any number of memory maps, i.e. visible in address space of some process. The sum of all pages currently loaded in memory is the page cache.
The pages are used to implement mmap interface and while regular read and write system calls can be implemented by the filesystem by other means, majority of interfaces uses generic function that also uses pages. There are generic functions, that when file read is requested allocate pages and call the filesystem to fill them in, one by one. For block-device-based filesystem, it just calculates appropriate addresses and delegates this filling to the block device driver.
ramdev (ramdisk)
Ramdev is regular block device. This allows layering any filesystem on top of it, but it is restricted by the block device interface. And that has just methods to fill in a page allocated by the caller and write it back. That’s exactly what is needed for real block devices like disks, memory cards, USB mass storage and such, but for ramdisk it means, that the data exist in memory twice, once in the memory of the ramdev and once in the memory allocated by the caller.
This is the old way of implementing initrd. From times when initrd was rare and exotic occurence.
tmpfs
Tmpfs is different. It’s a dummy filesystem. The methods it provides to VFS are the absolute bare minimum to make it work (as such it’s excellent documentation of what the inode, dentry and file methods should do). Files only exist if there is corresponding inode and dentry in the inode cache, created when the file is created and never expired unless the file is deleted. The pages are associated to files when data is written and otherwise behave as anonymous ones (data may be stored to swap, page structures remain in use as long as the file exists).
This means there are no extra copies of the data in memory and the whole thing is a lot simpler and due to that slightly faster too. It simply uses the data structures, that serve as cache for any other filesystem, as it’s primary storage.
This is the new way of implementing initrd (initramfs, but the image is still called just initrd).
It is also the way of implementing “posix shared memory” (which simply means tmpfs is mounted on /dev/shm and applications are free to create files there and mmap them; simple and efficient) and recently even /tmp and /run (or /var/run) often have tmpfs mounted especially on notebooks to keep disks from having to spin up or avoid some wear in case of SSDs.