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Initial ramdisk

January 01, 1970

In Linux systems, initrd (initial ramdisk) is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs (from INITial RAM File System) refer to two different methods of achieving this. Both are commonly used to make preparations before the real root file system can be mounted.

Rationale edit

Many Linux distributions ship a single, generic Linux kernel image – one that the distribution's developers create specifically to boot on a wide variety of hardware. The device drivers for this generic kernel image are included as loadable kernel modules because statically compiling many drivers into one kernel causes the kernel image to be much larger, perhaps too large to boot on computers with limited memory, or in some cases to cause boot-time crashes or other problems due to probing for nonexistent or conflicting hardware. This static-compiled kernel approach also leaves modules in kernel memory which are no longer used or needed, and raises the problem of detecting and loading the modules necessary to mount the root file system at boot time, or for that matter, deducing where or what the root file system is.[1]

To further complicate matters, the root file system may be on a software RAID volume, LVM, NFS (on diskless workstations), or on an encrypted partition. All of these require special preparations to mount.[2]

Another complication is kernel support for hibernation, which suspends the computer to disk by dumping an image of the entire contents of memory to a swap partition or a regular file, then powering off. On next boot, this image has to be made accessible before it can be loaded back into memory.

To avoid having to hardcode handling for so many special cases into the kernel, an initial boot stage with a temporary root file-system – now dubbed early user space – is used. This root file-system can contain user-space helpers which do the hardware detection, module loading and device discovery necessary to get the real root file-system mounted.[2]

Implementation edit

See also: Booting
See also: Linux startup process
 
mkinitcpio, a program to generate initramfs on Arch Linux and related distributions
 
dracut, another program to generate initramfs on some Linux distributions

An image of this initial root file system (along with the kernel image) must be stored somewhere accessible by the Linux bootloader or the boot firmware of the computer. This can be the root file system itself, a boot image on an optical disc, a small partition on a local disk (a boot partition, usually using ext2 or FAT file systems), or a TFTP server (on systems that can boot from Ethernet).

The bootloader will load the kernel and initial root file system image into memory and then start the kernel, passing in the memory address of the image. At the end of its boot sequence, the kernel tries to determine the format of the image from its first few blocks of data, which can lead either to the initrd or initramfs scheme.

In the initrd scheme, the image may be a file system image (optionally compressed), which is made available in a special block device (/dev/ram) that is then mounted as the initial root file system.[3] The driver for that file system must be compiled statically into the kernel. Many distributions originally used compressed ext2 file system images, while the others (including Debian 3.1) used cramfs in order to boot on memory-limited systems, since the cramfs image can be mounted in-place without requiring extra space for decompression. Once the initial root file system is up, the kernel executes /linuxrc as its first process;[4] when it exits, the kernel assumes that the real root file system has been mounted and executes /sbin/init to begin the normal user-space boot process.[3]

In the initramfs scheme (available since the Linux kernel 2.6.13), the image may be a cpio archive (optionally compressed). The archive is unpacked by the kernel into a special instance of a tmpfs that becomes the initial root file system. This scheme has the advantage of not requiring an intermediate file system or block drivers to be compiled into the kernel.[5] Some systems use the dracut package to create an initramfs image.[6] In the initramfs scheme, the kernel executes /init as its first process that is not expected to exit.[5] For some applications, initramfs can use the casper utility to create a writable environment using unionfs to overlay a persistence layer over a read-only root filesystem image. For example, overlay data can be stored on a USB flash drive, while a compressed SquashFS read-only image stored on a live CD acts as a root filesystem.[7][8]

Depending on which algorithms were compiled statically into it, the kernel can unpack initrd/initramfs images compressed with gzip, bzip2, LZMA, XZ, LZO, LZ4,[9] and zstd.

Mount preparations edit

Some Linux distributions such as Debian will generate a customized initrd image which contains only whatever is necessary to boot some particular computer, such as ATA, SCSI and filesystem kernel modules. These typically embed the location and type of the root file system.

Other Linux distributions (such as Fedora and Ubuntu) generate a more generic initrd image. These start only with the device name of the root file system (or its UUID) and must discover everything else at boot time. In this case, the software must perform a complex cascade of tasks to get the root file system mounted:

  • Any hardware drivers that the boot process depends on must be loaded. A common arrangement is to pack kernel modules for common storage devices onto the initrd and then invoke a hotplug agent to pull in modules matching the computer's detected hardware.
  • On systems which display a boot splash screen, the video hardware must be initialized and a user-space helper started to paint animations onto the display in lockstep with the boot process.
  • If the root file system is on NFS, it must then bring up the primary network interface, invoke a DHCP client, with which it can obtain a DHCP lease, extract the name of the NFS share and the address of the NFS server from the lease, and mount the NFS share.
  • If the root file system appears to be on a software RAID device, there is no way of knowing which devices the RAID volume spans; the standard MD utilities must be invoked to scan all available block devices and bring the required ones online.
  • If the root file system appears to be on a logical volume, the LVM utilities must be invoked to scan for and activate the volume group containing it.
  • If the root file system is on an encrypted block device, the software needs to invoke a helper script to prompt the user to type in a passphrase and/or insert a hardware token (such as a smart card or a USB security dongle), and then create a decryption target with the device mapper.

Some distributions use an event-driven hotplug agent such as udev, which invokes helper programs as hardware devices, disk partitions and storage volumes matching certain rules come online. This allows discovery to run in parallel, and to progressively cascade into arbitrary nestings of LVM, RAID or encryption to get at the root file system.

When the root file system finally becomes visible, any maintenance tasks that cannot run on a mounted root file system are done, the root file system is mounted read-only, and any processes that must continue running (such as the splash screen helper and its command FIFO) are hoisted into the newly mounted root file system.

The final root file system cannot simply be mounted over /, since that would make the scripts and tools on the initial root file system inaccessible for any final cleanup tasks:

  • On an initrd, the new root is mounted at a temporary mount point and rotated into place with pivot_root(8) (which was introduced specifically for this purpose). This leaves the initial root file system at a mount point (such as /initrd) where normal boot scripts can later unmount it to free up memory held by the initrd.
  • On an initramfs, the initial root file system cannot be rotated away.[10] Instead, it is simply emptied and the final root file system mounted over the top.

Most initial root file systems implement /linuxrc or /init as a shell script and thus include a minimal shell (usually /bin/ash) along with some essential user-space utilities (usually the BusyBox toolkit). To further save space, the shell, utilities and their supporting libraries are typically compiled with space optimizations enabled (such as with gcc's "-Os" flag) and linked against klibc, a minimal version of the C library written specifically for this purpose.[11]

Other uses edit

Installers for Linux distributions typically run entirely from an initramfs, as they must be able to host the installer interface and supporting tools before any persistent storage has been set up.[citation needed]

Tiny Core Linux[12] and Puppy Linux[13][failed verification] can run entirely from initrd.

Similarities in other operating systems edit

Since Windows Vista,[14] Windows can boot from a WIM disk image file, for which the file format is published;[15] it is similar to the ZIP format except that it supports hard links, deduplicated chunks, and uses chunk-by-chunk compression. In this case, the whole WIM is initially loaded into RAM, followed by the kernel initialisation. Next, the loaded WIM is available as a SystemRoot with an assigned drive letter. The Windows installer uses this so it boots from BOOT.WIM, and then uses INSTALL.WIM as the collection of the Windows files to be installed.

Also, Windows Preinstallation Environment (Windows PE) uses the same, being a base for separate-boot versions of some antivirus and backup/disaster recovery software.

It is also possible to install Windows so that it will always boot from a WIM or VHD file placed on a physical drive. However, this is rarely used since the Windows boot loader is capable of loading the .sys files for boot-time kernel modules itself, which is the task that requires initrd in Linux.

See also edit

References edit

  1. ^ Almesberger, Werner (2000), , Proceedings of the Ottawa Linux Symposium, archived from the original on 24 July 2008
  2. ^ a b Landley, Rob (15 March 2005), Introducing initramfs, a new model for initial RAM disks
  3. ^ a b Almesberger, Werner; Lermen, Hans (2000). . Archived from the original on 2 April 2015. Retrieved 14 March 2015.
  4. ^ "linux/do_mounts_initrd.c at 4f671fe2f9523a1ea206f63fe60a7c7b3a56d5c7 · torvalds/linux · GitHub". GitHub.
  5. ^ a b Landley, Rob (17 October 2005). "ramfs, rootfs, and initramfs docs, take 2". Linux kernel source tree.
  6. ^ Petersen, Richard Leland (2010). Fedora 13: Administration, Networking, Security. Alameda, California: Surfing Turtle Press. p. 76. ISBN 978-1-936280-02-5. Dracut uses kernel parameters listed on the GRUB kernel command line to configure the initramfs RAM file system on the fly, providing more flexibiltity and furthercutting down on RAM file system code.
  7. ^ "Ubuntu Manpage: casper - a hook for initramfs-tools to boot live systems". manpages.ubuntu.com.
  8. ^ Shawn Powers. "Casper, the Friendly (and Persistent) Ghost". Linux Journal. 2012.
  9. ^ Kyungsik Lee (30 May 2013). "LZ4 Compression and Improving Boot Time" (PDF). events.linuxfoundation.org. p. 18. Retrieved 29 May 2015.
  10. ^ Fish, Richard (6 July 2005). "pivot_root from initramfs causes circular reference in mount tree". Linux Kernel Bug Tracker. Retrieved 28 February 2009.
  11. ^ Garzik, Jeff (2 November 2002). "initramfs merge, part 1 of N". Linux kernel mailing list.
  12. ^ "Tiny Core Linux - Concepts". ibiblio.org.
  13. ^ Barry Kauler. "Puppy Linux Release Announcement". ibiblio.org.
  14. ^ "Windows Imaging File Format (WIM)". microsoft.com. Microsoft.
  15. ^ "Download Windows Imaging File Format (WIM) from Official Microsoft Download Center". Microsoft.com. Microsoft.

External links edit

  • Debian initramfs-tools
  • Detailed comparison of initrd-generating toolkits
  • Kernel documentation on early userspace support
  • "Motivation for switch from initrd to initramfs". Archived from the original on 4 January 2013. Alt URL

January 01, 1970
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In Linux systems initrd initial ramdisk is a scheme for loading a temporary root file system into memory to be used as part of the Linux startup process initrd and initramfs from INITial RAM File System refer to two different methods of achieving this Both are commonly used to make preparations before the real root file system can be mounted Contents 1 Rationale 2 Implementation 2 1 Mount preparations 2 2 Other uses 2 3 Similarities in other operating systems 3 See also 4 References 5 External linksRationale editMany Linux distributions ship a single generic Linux kernel image one that the distribution s developers create specifically to boot on a wide variety of hardware The device drivers for this generic kernel image are included as loadable kernel modules because statically compiling many drivers into one kernel causes the kernel image to be much larger perhaps too large to boot on computers with limited memory or in some cases to cause boot time crashes or other problems due to probing for nonexistent or conflicting hardware This static compiled kernel approach also leaves modules in kernel memory which are no longer used or needed and raises the problem of detecting and loading the modules necessary to mount the root file system at boot time or for that matter deducing where or what the root file system is 1 To further complicate matters the root file system may be on a software RAID volume LVM NFS on diskless workstations or on an encrypted partition All of these require special preparations to mount 2 Another complication is kernel support for hibernation which suspends the computer to disk by dumping an image of the entire contents of memory to a swap partition or a regular file then powering off On next boot this image has to be made accessible before it can be loaded back into memory To avoid having to hardcode handling for so many special cases into the kernel an initial boot stage with a temporary root file system now dubbed early user space is used This root file system can contain user space helpers which do the hardware detection module loading and device discovery necessary to get the real root file system mounted 2 Implementation editSee also Booting See also Linux startup process nbsp mkinitcpio a program to generate initramfs on Arch Linux and related distributions nbsp dracut another program to generate initramfs on some Linux distributions An image of this initial root file system along with the kernel image must be stored somewhere accessible by the Linux bootloader or the boot firmware of the computer This can be the root file system itself a boot image on an optical disc a small partition on a local disk a boot partition usually using ext2 or FAT file systems or a TFTP server on systems that can boot from Ethernet The bootloader will load the kernel and initial root file system image into memory and then start the kernel passing in the memory address of the image At the end of its boot sequence the kernel tries to determine the format of the image from its first few blocks of data which can lead either to the initrd or initramfs scheme In the initrd scheme the image may be a file system image optionally compressed which is made available in a special block device dev ram that is then mounted as the initial root file system 3 The driver for that file system must be compiled statically into the kernel Many distributions originally used compressed ext2 file system images while the others including Debian 3 1 used cramfs in order to boot on memory limited systems since the cramfs image can be mounted in place without requiring extra space for decompression Once the initial root file system is up the kernel executes linuxrc as its first process 4 when it exits the kernel assumes that the real root file system has been mounted and executes sbin init to begin the normal user space boot process 3 In the initramfs scheme available since the Linux kernel 2 6 13 the image may be a cpio archive optionally compressed The archive is unpacked by the kernel into a special instance of a tmpfs that becomes the initial root file system This scheme has the advantage of not requiring an intermediate file system or block drivers to be compiled into the kernel 5 Some systems use the dracut package to create an initramfs image 6 In the initramfs scheme the kernel executes init as its first process that is not expected to exit 5 For some applications initramfs can use the casper utility to create a writable environment using unionfs to overlay a persistence layer over a read only root filesystem image For example overlay data can be stored on a USB flash drive while a compressed SquashFS read only image stored on a live CD acts as a root filesystem 7 8 Depending on which algorithms were compiled statically into it the kernel can unpack initrd initramfs images compressed with gzip bzip2 LZMA XZ LZO LZ4 9 and zstd Mount preparations edit Some Linux distributions such as Debian will generate a customized initrd image which contains only whatever is necessary to boot some particular computer such as ATA SCSI and filesystem kernel modules These typically embed the location and type of the root file system Other Linux distributions such as Fedora and Ubuntu generate a more generic initrd image These start only with the device name of the root file system or its UUID and must discover everything else at boot time In this case the software must perform a complex cascade of tasks to get the root file system mounted Any hardware drivers that the boot process depends on must be loaded A common arrangement is to pack kernel modules for common storage devices onto the initrd and then invoke a hotplug agent to pull in modules matching the computer s detected hardware On systems which display a boot splash screen the video hardware must be initialized and a user space helper started to paint animations onto the display in lockstep with the boot process If the root file system is on NFS it must then bring up the primary network interface invoke a DHCP client with which it can obtain a DHCP lease extract the name of the NFS share and the address of the NFS server from the lease and mount the NFS share If the root file system appears to be on a software RAID device there is no way of knowing which devices the RAID volume spans the standard MD utilities must be invoked to scan all available block devices and bring the required ones online If the root file system appears to be on a logical volume the LVM utilities must be invoked to scan for and activate the volume group containing it If the root file system is on an encrypted block device the software needs to invoke a helper script to prompt the user to type in a passphrase and or insert a hardware token such as a smart card or a USB security dongle and then create a decryption target with the device mapper Some distributions use an event driven hotplug agent such as udev which invokes helper programs as hardware devices disk partitions and storage volumes matching certain rules come online This allows discovery to run in parallel and to progressively cascade into arbitrary nestings of LVM RAID or encryption to get at the root file system When the root file system finally becomes visible any maintenance tasks that cannot run on a mounted root file system are done the root file system is mounted read only and any processes that must continue running such as the splash screen helper and its command FIFO are hoisted into the newly mounted root file system The final root file system cannot simply be mounted over since that would make the scripts and tools on the initial root file system inaccessible for any final cleanup tasks On an initrd the new root is mounted at a temporary mount point and rotated into place with pivot root 8 which was introduced specifically for this purpose This leaves the initial root file system at a mount point such as initrd where normal boot scripts can later unmount it to free up memory held by the initrd On an initramfs the initial root file system cannot be rotated away 10 Instead it is simply emptied and the final root file system mounted over the top Most initial root file systems implement linuxrc or init as a shell script and thus include a minimal shell usually bin ash along with some essential user space utilities usually the BusyBox toolkit To further save space the shell utilities and their supporting libraries are typically compiled with space optimizations enabled such as with gcc s Os flag and linked against klibc a minimal version of the C library written specifically for this purpose 11 Other uses edit Installers for Linux distributions typically run entirely from an initramfs as they must be able to host the installer interface and supporting tools before any persistent storage has been set up citation needed Tiny Core Linux 12 and Puppy Linux 13 failed verification can run entirely from initrd Similarities in other operating systems edit This section needs additional citations for verification Please help improve this article by adding citations to reliable sources in this section Unsourced material may be challenged and removed August 2015 Learn how and when to remove this message Since Windows Vista 14 Windows can boot from a WIM disk image file for which the file format is published 15 it is similar to the ZIP format except that it supports hard links deduplicated chunks and uses chunk by chunk compression In this case the whole WIM is initially loaded into RAM followed by the kernel initialisation Next the loaded WIM is available as a SystemRoot with an assigned drive letter The Windows installer uses this so it boots from BOOT WIM and then uses INSTALL WIM as the collection of the Windows files to be installed Also Windows Preinstallation Environment Windows PE uses the same being a base for separate boot versions of some antivirus and backup disaster recovery software It is also possible to install Windows so that it will always boot from a WIM or VHD file placed on a physical drive However this is rarely used since the Windows boot loader is capable of loading the sys files for boot time kernel modules itself which is the task that requires initrd in Linux See also edit nbsp Linux portal dracut booster initramfs generator Linux startup process List of Linux distributions that run from RAM EFISTUB booting allows EFI firmware to load a Linux kernel Arch Linux Wiki References edit Almesberger Werner 2000 Booting linux the history and the future Proceedings of the Ottawa Linux Symposium archived from the original on 24 July 2008 a b Landley Rob 15 March 2005 Introducing initramfs a new model for initial RAM disks a b Almesberger Werner Lermen Hans 2000 Using the initial RAM disk initrd Archived from the original on 2 April 2015 Retrieved 14 March 2015 linux do mounts initrd c at 4f671fe2f9523a1ea206f63fe60a7c7b3a56d5c7 torvalds linux GitHub GitHub a b Landley Rob 17 October 2005 ramfs rootfs and initramfs docs take 2 Linux kernel source tree Petersen Richard Leland 2010 Fedora 13 Administration Networking Security Alameda California Surfing Turtle Press p 76 ISBN 978 1 936280 02 5 Dracut uses kernel parameters listed on the GRUB kernel command line to configure the initramfs RAM file system on the fly providing more flexibiltity and furthercutting down on RAM file system code Ubuntu Manpage casper a hook for initramfs tools to boot live systems manpages ubuntu com Shawn Powers Casper the Friendly and Persistent Ghost Linux Journal 2012 Kyungsik Lee 30 May 2013 LZ4 Compression and Improving Boot Time PDF events linuxfoundation org p 18 Retrieved 29 May 2015 Fish Richard 6 July 2005 pivot root from initramfs causes circular reference in mount tree Linux Kernel Bug Tracker Retrieved 28 February 2009 Garzik Jeff 2 November 2002 initramfs merge part 1 of N Linux kernel mailing list Tiny Core Linux Concepts ibiblio org Barry Kauler Puppy Linux Release Announcement ibiblio org Windows Imaging File Format WIM microsoft com Microsoft Download Windows Imaging File Format WIM from Official Microsoft Download Center Microsoft com Microsoft External links editDebian initramfs tools Detailed comparison of initrd generating toolkits Kernel documentation on early userspace support Motivation for switch from initrd to initramfs Archived from the original on 4 January 2013 Alt URL Retrieved from https en wikipedia org w index php title Initial ramdisk amp oldid 1222499088, wikipedia, wiki, book, books, library,

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