- klibc, a userspace C library, currently packaged separately, that is
optimized for correctness and small size.
-The cpio file format used by initramfs is the "newc" (aka "cpio -c")
+The cpio file format used by initramfs is the "newc" (aka "cpio -H newc")
format, and is documented in the file "buffer-format.txt". There are
two ways to add an early userspace image: specify an existing cpio
archive to be used as the image or have the kernel build process build
CONFIG_INITRAMFS_SOURCE. Sources can be either directories or files -
cpio archives are *not* allowed when building from sources.
-A source directory will have it and all of it's contents packaged. The
+A source directory will have it and all of its contents packaged. The
specified directory name will be mapped to '/'. When packaging a
directory, limited user and group ID translation can be performed.
INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
initrd format, an cpio archive. It must be called "/init". This binary
is responsible to do all the things prepare_namespace() would do.
- To remain backwards compatibility, the /init binary will only run if it
+ To maintain backwards compatibility, the /init binary will only run if it
comes via an initramfs cpio archive. If this is not the case,
init/main.c:init() will run prepare_namespace() to mount the final root
and exec one of the predefined init binaries.
Ramfs is a very simple filesystem that exports Linux's disk caching
mechanisms (the page cache and dentry cache) as a dynamically resizable
-ram-based filesystem.
+RAM-based filesystem.
Normally all files are cached in memory by Linux. Pages of data read from
backing store (usually the block device the filesystem is mounted on) are kept
------------------
The older "ram disk" mechanism created a synthetic block device out of
-an area of ram and used it as backing store for a filesystem. This block
+an area of RAM and used it as backing store for a filesystem. This block
device was of fixed size, so the filesystem mounted on it was of fixed
size. Using a ram disk also required unnecessarily copying memory from the
fake block device into the page cache (and copying changes back out), as well
to avoid this copying by playing with the page tables, but they're unpleasantly
complicated and turn out to be about as expensive as the copying anyway.)
More to the point, all the work ramfs is doing has to happen _anyway_,
-since all file access goes through the page and dentry caches. The ram
-disk is simply unnecessary, ramfs is internally much simpler.
+since all file access goes through the page and dentry caches. The RAM
+disk is simply unnecessary; ramfs is internally much simpler.
Another reason ramdisks are semi-obsolete is that the introduction of
loopback devices offered a more flexible and convenient way to create
initramfs archive is a gzipped cpio archive (like tar only simpler,
see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
kernel's cpio extraction code is not only extremely small, it's also
- __init data that can be discarded during the boot process.
+ __init text and data that can be discarded during the boot process.
- The program run by the old initrd (which was called /initrd, not /init) did
some setup and then returned to the kernel, while the init program from
non-GPL code you'd like to run from initramfs, without conflating it with
the GPL licensed Linux kernel binary).
-It can also be used to supplement the kernel's built-in initamfs image. The
+It can also be used to supplement the kernel's built-in initramfs image. The
files in the external archive will overwrite any conflicting files in
the built-in initramfs archive. Some distributors also prefer to customize
a single kernel image with task-specific initramfs images, without recompiling.
The move to early userspace is necessary because finding and mounting the real
root device is complex. Root partitions can span multiple devices (raid or
separate journal). They can be out on the network (requiring dhcp, setting a
-specific mac address, logging into a server, etc). They can live on removable
+specific MAC address, logging into a server, etc). They can live on removable
media, with dynamically allocated major/minor numbers and persistent naming
issues requiring a full udev implementation to sort out. They can be
compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
----------------------
Recent kernels have support for populating a ramdisk from a compressed cpio
-archive, on such systems, the creation of a ramdisk image doesn't need to
-involve special block devices or loopbacks, you merely create a directory on
+archive. On such systems, the creation of a ramdisk image doesn't need to
+involve special block devices or loopbacks; you merely create a directory on
disk with the desired initrd content, cd to that directory, and run (as an
example):
generated with all the necessary modules. Then, only /sbin/init or a file
read by it would have to be different.
-A third scenario are more convenient recovery disks, because information
+A third scenario is more convenient recovery disks, because information
like the location of the root FS partition doesn't have to be provided at
boot time, but the system loaded from initrd can invoke a user-friendly
dialog and it can also perform some sanity checks (or even some form of
Mixed change_root and pivot_root mechanism
------------------------------------------
-In case you did not want to use root=/dev/ram0 to trig the pivot_root mechanism,
-you may create both /linuxrc and /sbin/init in your initrd image.
+In case you did not want to use root=/dev/ram0 to trigger the pivot_root
+mechanism, you may create both /linuxrc and /sbin/init in your initrd image.
/linuxrc would contain only the following:
umount -n /proc
Once linuxrc exited, the kernel would mount again your initrd as root,
-this time executing /sbin/init. Again, it would be duty of this init
+this time executing /sbin/init. Again, it would be the duty of this init
to build the right environment (maybe using the root= device passed on
the cmdline) before the final execution of the real /sbin/init.
RAM from the buffer cache. The driver marks the buffers it is using as dirty
so that the VM subsystem does not try to reclaim them later.
-Also, the RAM disk supports up to 16 RAM disks out of the box, and can
-be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS"
-in drivers/block/rd.c. To use RAM disk support with your system, run
-'./MAKEDEV ram' from the /dev directory. RAM disks are all major number 1, and
-start with minor number 0 for /dev/ram0, etc. If used, modern kernels use
-/dev/ram0 for an initrd.
+The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
+to support an unlimited number of RAM disks (at your own risk). Just change
+the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
+and (re)build the kernel.
+
+To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
+directory. RAM disks are all major number 1, and start with minor number 0
+for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
The old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to
make it clearer. The original "ramdisk=<ram_size>" has been kept around for
compatibility reasons, but it may be removed in the future.
+There are also config symbols (in the Block drivers config menu) for these
+variables: BLK_DEV_RAM_SIZE defaults to 4096 and BLK_DEV_RAM_BLOCKSIZE
+defaults to 1024.
The new RAM disk also has the ability to load compressed RAM disk images,
allowing one to squeeze more programs onto an average installation or
projects / GitHub / MotorolaMobilityLLC / kernel-slsi.git / commitdiff