mirror of
https://github.com/adulau/aha.git
synced 2024-12-28 11:46:19 +00:00
docs: ramdisk/initrd/initramfs corrections
initrd/initramfs/ramdisk docs: - fix typos/spellos/grammar - clarify RAM disk config location - correct cpio option Acked-by: Bryan O'Sullivan <bos@serpentine.com> Acked-by: Rob Landley <rob@landley.net> Cc: Werner Almesberger <werner@almesberger.net> Cc: H. Peter Anvin <hpa@zytor.com> Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com> Acked-by: Jesper Juhl <jesper.juhl@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Randy Dunlap
Linus Torvalds
parent
0e1ccb9619
commit
1810732e94
4 changed files with 27 additions and 22 deletions
|
|
@ -19,7 +19,7 @@ It consists of several major infrastructure components:
|
|||
- 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
|
||||
|
|
@ -44,7 +44,7 @@ The image is specified as one or more sources in
|
|||
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
|
||||
|
|
@ -144,7 +144,7 @@ c) using initramfs. The call to prepare_namespace() must be skipped.
|
|||
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.
|
||||
|
|
|
|||
|
|
@ -8,7 +8,7 @@ What is ramfs?
|
|||
|
||||
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
|
||||
|
|
@ -34,7 +34,7 @@ ramfs and ramdisk:
|
|||
------------------
|
||||
|
||||
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
|
||||
|
|
@ -46,8 +46,8 @@ unnecessary work for the CPU, and pollutes the CPU caches. (There are tricks
|
|||
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
|
||||
|
|
@ -103,7 +103,7 @@ All this differs from the old initrd in several ways:
|
|||
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
|
||||
|
|
@ -220,7 +220,7 @@ device) but the separate packaging of initrd (which is nice if you have
|
|||
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.
|
||||
|
|
@ -339,7 +339,7 @@ smooth transition and allowing early boot functionality to gradually move to
|
|||
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,
|
||||
|
|
|
|||
|
|
@ -80,8 +80,8 @@ Compressed cpio images
|
|||
----------------------
|
||||
|
||||
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):
|
||||
|
||||
|
|
@ -293,7 +293,7 @@ information as small as possible. In this case, a common initrd could be
|
|||
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
|
||||
|
|
@ -339,8 +339,8 @@ the new, supported mechanism is called "pivot_root".
|
|||
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:
|
||||
|
||||
|
|
@ -350,7 +350,7 @@ echo 0x0100 >/proc/sys/kernel/real-root-dev
|
|||
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.
|
||||
|
||||
|
|
|
|||
|
|
@ -22,16 +22,21 @@ The RAM disk dynamically grows as more space is required. It does this by using
|
|||
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
|
||||
|
|
|
|||
Loading…
Reference in a new issue