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Category: Initramfs cpio

Initramfs cpio

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After kernel booted, it tries to mount a file system. Using Linux on DaVinci, there are several options where this file system can come from. Options are. The last option, an initial file system in a ram disk is called initrd init ial r am d isk. Note that using an initrd with recent kernels is still possible and has some advantagesbut isn't recommended any more. Using initramfs is the preferred way today.

The two other options above hard disk and flash file system give the target system permanently access to its file system without any external debug connection e. Note: Change count to your required filesystem size. To fill empty ramdisk created above with all files needed for ramdisk, mount the image and fill it. Content would be e. The resulting ramdisk.

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Now, you can install the ramdisk via u-boot e. For this copy filled ramdisk created above to your tftpboot directory on host e. Then start target and copy the data into RAM and flash:.

Note: Replace filesize above by the value the tftp download command gives you as Bytes transferred. Note: In example above with "8M" we assume that your ramdisk is 8MBytes. Adapt this to your needs. To use initramfs a cpio archive is embedded directly into the kernel. Instead, the initial file system is directly incorporated into the kernel. With this, the kernel size increases by the file system size. It's like you embed above ramdisk directly into the kernel.

Cause initramfs is directly embedded in the the kernel, its creation is simpler. You simply have to fill a directory on your host with the target filesystem you like and then pass the path to this directory to the kernel build process. Note: cpio system used for initramfs can't handle hard links.

If you e. Note: To be able to detect initramfs by kernel properly, the top level directory has to contain a program called init. The only difference from creating an initrd is to give the kernel the path to the target file system you like to embed:.

Then, if you compile the kernel, e. No special installation like above with initrd is necessary. The initramfs is already in the kernel. If you start the kernel, the initramfs is already there.

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I am making my own initramfs following the Gentoo wiki. Instead of the familiar tar and gzipthe page is telling me to use cpio and gzip. Wikipedia says that cpio is used by the 2. Is this just a convention or is cpio better for initramfs?

Can I still use tar and gzip? Here's a Linux Journal article about it from But that says nothing either way about the archive format, and there are alternative tools, such as:. The complete initramfs archive format is explained in buffer-format. All three together come to less than 26k total of human-readable text.

Linux is not part of either, and is free to make its own technical decisions. The kernel provides its own tools to create and extract this format anyway. Using an existing standard was preferable, but not essential.

From what I remember of my old SysV days, cpio could handle dev files, but tar could not; this made cpio the 'raw' backup utility of choice before dump came around. It was also easier to handle partial filesets and hard links so incremental backups were easier. I think that GNU tar has caught up with cpio features so now it is just a matter of user comfortability.

Both cpio and tar should be installed by default. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Why use cpio for initramfs? Ask Question. Asked 9 years, 2 months ago.

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Active 9 months ago. Viewed 3k times. BTW using Gentoo I find much easier to configure built-in initramfs rather then hand-made one. You misunderstood me.Initial RAM file system, abbreviated as initramfs, allows the kernel to run user space applications from a RAM disk built-in into the kernel or passed by the boot loader.

initramfs cpio

Introduced in Linux 2. After extracting, the kernel checks to see if rootfs contains a file "init", and if so it executes it as PID 1. If found, this init process is responsible for bringing the system the rest of the way up, including locating and mounting the real root device if any.

initramfs cpio

Traditionally, initramfs is meant to be a small root filesystem only capable of finding and hand over to the main rootfs. It allows to specify an image to be used as initramfs while the main rootfs is still built. However, for embedded systems initramfs is often used as the main rootfs on its own. The main difference is that the traditional use case has an init script that mounts the final image, whereas for the embedded case the init script starts the application s that the device should run on its own and does not hand over to another rootfs.

The rest of the article will focus on this embedded scenario. To configure the build environment please refer to the article OpenEmbedded core. A minimal initramfs root filesystem will be generated using OpenEmbedded and then built into the kernel. First, build the image recipe named initramfs-debug-image. Remember to configure the target machine in the local.

Building this image results in a cpio compressed root filesystem named the following is the example initramfs for the Colibri VF61, the name will change for other machines and image versions :. By adding the following to your local.

There are two possible ways to deploy the initramfs: into the kernel or as an external file. While having the initramfs into the kernel is simple, having an external initramfs can be useful do to the flexibility of being able to substitute it without recompiling the kernel itself. If there is a internal and external initramfs, the external initramfs will extracted last and hence will overwrites files from the internal initramfs. That can be used to add or change functionality of a built-in initramfs.

For details about configuring the environment for building kernel from source, please refer to the article Build U-Boot and Linux Kernel from Source Code. To flash the image to the module, follow the steps from here. The file generated by OpenEmbedded is in the format. The first step is to wrap it so U-Boot is able to use the initramfs:. Copy the image to an SD card, insert it into the carrier board, power-on and stop U-Boot.

Linux cpio Examples: How to Create and Extract cpio Archives (and tar archives)

Then load the image to the RAM and write it to the module's non-volatile memory:. Some environment variables from U-Boot need to be configured before the module boot to the initramfs image at startup:.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Server Fault is a question and answer site for system and network administrators.

It only takes a minute to sign up. So cpio extracts only. But lsinitramfs show much more files inside it:. The first image contains just the microcode.

The second contains everything else. Your cpio command just extracts the first one. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asked 2 years, 6 months ago. Active 1 year, 2 months ago. Viewed 2k times. Selivanov Pavel Selivanov Pavel 1, 1 1 gold badge 20 20 silver badges 44 44 bronze badges. Just googled the same problem and found an answer unix.

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Sergey Thanks! May be you can wtire this as an answer so I can accept it? Active Oldest Votes. Benjamin Drung Benjamin Drung 81 3 3 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.

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Hot Network Questions. Question feed. Server Fault works best with JavaScript enabled.Some Linux-based computer systems require an initramfs to boot properly. This guide covers the concepts of the initramfs as well as how to properly create and manage initramfs instances. For many users, an initramfs system is of no concern. Their system uses a simple partitioning schema with no exotic drivers or setups like encrypted file systemsso the Linux kernel is entirely capable to hand over control to the init binary on their system.

But for many systems, an initramfs is mandatory. The key concept to understanding what an initramfs is or is needed for is to understand how the Linux boot process works, even in a high-level approach. Once the Linux kernel has control over the system which it gets after being loaded by the boot loaderit prepares its memory structures and drivers. It then hands over control to an application usually init whose task it is to further prepare the system and make sure that, at the end of the boot process, all necessary services are running and the user is able to log on.

The init application does that by launching, among other services, the udev daemon who will further load up and prepare the system based on the detected devices. When udev is launched, all remaining file systems that have not been mounted are mounted, and the remainder of services is started.

For systems where all necessary files and tools reside on the same file system, the init application can perfectly control the further boot process. But when multiple file systems are defined or more exotic installations are donethis might become a bit more tricky:. The initrd is an in-memory disk structure ramdisk that contains the necessary tools and scripts to mount the needed file systems before control is handed over to the init application on the root file system.

The Linux kernel triggers the setup script usually called linuxrc but that name is not mandatory on this root disk, which prepares the system, switches to the real root file system and then calls init. An initramfs is an initial ram file system based on tmpfs a size-flexible, in-memory lightweight file systemwhich also did not use a separate block device so no caching was done and all overhead mentioned earlier disappears. Just like the initrd, it contains the tools and scripts needed to mount the file systems before the init binary on the real root file system is called.

These tools can be decryption abstraction layers for encrypted file systemslogical volume managers, software raid, bluetooth driver based file system loaders, etc. The content of the initramfs is made by creating a cpio archive. The choice of cpio here was because it was easier to implement code-wise and supported device files which tar did not support at the time.

All files, tools, libraries, configuration settings if applicableetc. This archive is then compressed using the gzip utility and stored alongside the Linux kernel. The boot loader will then offer it to the Linux kernel at boot time so the kernel knows an initramfs is needed. Once detected, the Linux kernel will create a tmpfs file system, extract the contents of the archive on it, and then launch the init script located in the root of the tmpfs file system.

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This script then mounts the real root file system after making sure it can mount it, for instance by loading additional modules, preparing an encryption abstraction layer, etc. To create an initramfs, it is important to know what additional drivers, scripts and tools will be needed to boot the system.As seen above, the ls command passes the three object filenames to cpio command and cpio generates the object.

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We already know how to use the tar command effectively. As seen above, instead of redirecting the standard output you can mention the output archive filename with the option -F. The modification time of the files can be preserved when we are restoring the cpio archive files as shown below.

How to View, Modify and Recreate initrd. It reads the file list from the standard input and pass it to the target directory. Tagged as: cpio rpmcpio vs tar. Pankaj: why is cpio better than tar? Carsten: this post is from — since then all these limitations have been fixed I believe, e. Notify me of followup comments via e-mail.

What is the difference between initrd and initramfs? (2 Solutions!!)

All rights reserved Terms of Service. Thanks for such a great article. Jufa August 26,pm. Great examples. How about examples of sending the archive to remote tape drive in one line? Pankaj September 2,am. Sandeep Maurya November 25,am.

Carsten December 26,pm. Is any way to copy files via ssh using cpio? F0bos May 30,am. Drink May 14,am. I will be posting instruction guides, how-to, troubleshooting tips and tricks on Linux, database, hardware, security and web. My focus is to write articles that will either teach you or help you resolve a problem. Read more about Ramesh Natarajan and the blog.

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I would like to activate plymouth in my Embedded board Raspberry Pi with a Yocto-based Linux distribution. However I am not sure how to mount the cpio. What I've tried so far involves both of these approaches.

I copy the cpio.

initramfs cpio

To break it down, here is how it looks:. However, nothing is enough for the kernel to even give an error. That means, system boots up as normal, and there is no sign of initramfs usage. I don't know what I'm missing here. Anything that catches your attention, please let me know. Any guidance is well appreciated. I was able to boot into the initramfs by copying the resulting. Then I've edited the config. Learn more. Asked 1 year, 10 months ago. Active 1 year, 8 months ago.

Viewed 2k times. Using Yocto-based tools, I'm able to generate several files for deployment. These involve: sdimg file for writing to the SD card A cpio. Now, in config. I just noticed something weird. Initramfs should be before rootfs is mounted to I'll have to move my cpio. I think this is the right way to proceed. I'll update the question afterwards.

Still the same behavior. Boots up as usual, but no indication of initramfs. There are two ways to pass the cpio archive to the kernel for the initramfs, and apparently you're using neither method. The alternative is to pass the address of the cpio archive in place of the initrd image during the kernel start.

Will update it. Did you figure this out? I am trying to do the same thing. Active Oldest Votes. Nanowire Nanowire 11 1 1 bronze badge.

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