Ramdisk

Kernel | Top | ZOID

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Introduction

Both the CN and ION Linux kernels require a ramdisk to boot. Ramdisk images contain minimal Linux utilities, init scripts, configuration files, kernel modules, etc, which are required by the OS boot process.

ION ramdisk is an ELF file that contains a cpio archive of system files. Two ION ramdisk images are currently generated:

BGP-ION-ramdisk-for-CNL.elf

Default ION ramdisk for ZeptoOS.

BGP-ION-ramdisk-for-CNK.elf

Use this one if you need to run IBM CNK on the compute nodes (uses IBM CIOD instead of ZOID)

Our ION ramdisks are similar to the default ION ramdisk from IBM, but we add some extra files to support ZeptoOS features. The extra files are located in ramdisk/ION/ramdisk-add/. The build-ramdisk script from IBM BGP driver is used to create the ION ramdisks.

The CN ramdisk is also a gzip'ed cpio archive of system files, but CN ramdisk is embedded into the CN kernel image (BGP-CN-zImage-with-initrd.elf). The CN ramdisk is created by a custom ramdisk build script (ramdisk/CN/create-bgp-cn-linux-ramdisk.pl). Both build-ramdisk and create-bgp-cn-linux-ramdisk.pl are wrappers of the Linux kernel's gen_init_cpio command.

Creating ramdisk images

The ramdisk images are always (re-)created from prebuild objects if one types make at the top level directory (without any make target).

If one wants to create an ION ramdisk individually (without rebuilding other images), type:

$ make bgp-ion-ramdisk-cnl

If one wants to create a CN ramdisk (technically, create a CN kernel image with new ramdisk contents), type:

$ make bgp-cn-linux

Note: the newly built CN ramdisk can be found in ramdisk/CN/bgp-cn-ramdisk.cpio.gz, but it is not useable until it is embedded into the kernel image.

For other ramdisk-related make targets, please refer to Configuration.

Modifying ramdisk contents

You can customize ramdisk contents for your purpose, i.e., debugging, running your custom system software on BGP, etc.

CN ramdisk

The CN ramdisk can be customized by editing the CN ramdisk build script, which is ramdisk/CN/create-bgp-cn-linux-ramdisk.pl. The build script allows to set the permission bits, create device files, etc.

Most of the contents of the CN ramdisk is kept in ramdisk/CN/tree/, but this is not a hard rule. Source files can reside anywhere as long as they are accessible from the script. It may be possible to use binaries and libraries from the login nodes, as long as they are a 32-bit PPC files (use the file command to verify) and all their dependencies are also copied.

Here is a practical example. Suppose that you need the od command in CN ramdisk. You could build the command from source code, but if you want to do something quick, you can try using the login node's version:

$ file /usr/bin/od
/usr/bin/od: ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV),
for GNU/Linux 2.6.4, dynamically linked (uses shared libs), for GNU/Linux 2.6.4, stripped
$ ldd /usr/bin/od
linux-vdso32.so.1 =>  (0x00100000)
libc.so.6 => /lib/ppc970/libc.so.6 (0x0fe8b000)
/lib/ld.so.1 (0xf7fe1000)

It is a 32-bit PPC executable and the current CN ramdisk has all the necessary shared libraries, so it can be used. Now add the command to a perl array named @cmdlists in ramdisk/CN/create-bgp-cn-linux-ramdisk.pl script and type make to recreate the CN ramdisk:

$ vi ramdisk/CN/create-bgp-cn-linux-ramdisk.pl
# add the following line to @cmdlists 
     "file /bin/od   /usr/bin/od 0755  0  0",
$ make bgp-cn-linux

Now the CN ramdisk has /bin/od with file permissions 0755, uid=0, and gid=0.

The added line is a command for the gen_init_cpio tool. One can also create directories, device files, symbolick links, pipe files, socket files, etc:

file <name> <location> <mode> <uid> <gid>
dir <name> <mode> <uid> <gid>
nod <name> <mode> <uid> <gid> <dev_type> <maj> <min>
slink <name> <target> <mode> <uid> <gid>
pipe <name> <mode> <uid> <gid>
sock <name> <mode> <uid> <gid>

<name>      name of the file/dir/nod/etc in the archive
<location>  location of the file in the current filesystem
<target>    link target
<mode>      mode/permissions of the file
<uid>       user id (0=root)
<gid>       group id (0=root)
<dev_type>  device type (b=block, c=character)
<maj>       major number of nod
<min>       minor number of nod

The order of the commands in @cmdlists matters. They are executed from top to bottom, so one cannot add a file to a directory that has not yet been created.

CN Linux startup script

The first thing that the Linux kernel does after it boots is to execute the init program. The init program is usually in /sbin/, and in the CN ramdisk case it is part of the busybox. init reads in a config file from /etc/inittab, which in our case instructs it to execute the /etc/init.d/rc.sysinit startup script.

Our startup script is very minimalistic; its two most important actions are to start the telnet daemon to allow users to login from the I/O nodes and then to start the ZOID control process which takes care of IP forwarding and job control.

In case you need to start some process at the CN boot time, you can add its invocations to ramdisk/CN/tree/etc/init.d/rc.sysinit, before /sbin/control is invoked.

ION ramdisk

Unlike with the CN ramdisk, the range of customization is limited on the ION ramdisk. There is no control over file permission bits, one cannot create device nodes, etc. Currently we build the ION ramdisk using IBM's build-ramdisk script by specifying an add-on tree which contains our extra files.

What you can do are basically:

• add files,
• overwrite default ramdisk files by adding custom files with the same names.

Once files have been added under ramdisk/ION/ramdisk-add/, they will be automatically added to the ramdisk on the next rebuild. Here is an example of how to add a file to the ION ramdisk:

$ vi ramdisk/ION/ramdisk-add/etc/yourfile
$ make bgp-ion-ramdisk-cnl

If you need more than file adding, you might need to edit the build-ramdisk script itself. The script is located in /bgsys/drivers/ppcfloor/. Copy the script to your working directory, edit it and change the script path in ramdisk/ION/Makefile.

ION startup script

There is no rc.sysinit in ramdisk/ION/ramdisk-add/ since rc.sysinit is provided from IBM ramdisk tree. i.e., /bgsys/drivers/ppcfloor/ramdisk/etc/init.d/rc.sysinit is default one. You can copy the default one to ramdisk/etc/init.d/rc.sysinit (local) and modify it to change the startup behaviour but it is not recommended.

In most cases, what you need is to start your software at ION boot time. For such purpose, you can add your ION RC script to ramdisk-add/etc/init.d/rc3.d to do some action.

RC script has own naming convention.

• S##xxxx : boot time scripts
• K##xxxx : shut down scripts

It starts with S or K. Scritps with S are boot time script and scripts with K are shut down scripts. A two-digit number is followed by 'S' or 'K' is used to decide execution order ; a smaller number script is executed before a larger number script. Then script name follows. The init scripts passes "start" as the 1st argument to boot time scripts when it is executed. Similarly, "stop" is passed to shut down script. If you parse the argument, one rc script can serve as both boot time and shut down script. Here is a template of rc script.

#!/bin/sh
. /etc/rc.status

rc_reset
case "$1" in
    start)
        # fill here #
        ;;
    stop)
        # fill here #
        ;;
    restart)
        # fill here #
        ;;
    status)
        # fill here #
        ;;
    *)
	echo "Usage: $0 {start|stop|restart|status}"
	exit 1
	;;
esac
rc_exit

Default Zepto ION ramdisk contains the following rc scripts.

boot scripts

S00zepto
S01bootsysctl
S02syslog
S05ntp
S11sshd
S12zepto
S40gpfs
S43ibmcmp
S46essl
S50ciod
S51zoid
S99zepto

shutdown scripts

K05ntp
K10sshd
K15ciod
K20gpfs
K30syslog
K50bgsys.64

Ramdisk size limitation

On regular Linux environment, ramdisk size is basically limited by free memory size at the time when ramdisk is loaded into memory. However, on BGP, the system software(non-opensource) can not handle bigger image. We don't have the exact number on the boot image size limitation but 100MB or bigger ramdisk might fail to boot with the current environment. If you add bigger files to ramdisk, please make sure the ramdisk file size, specifically, BGP-ION-ramdisk-for-CNL.elf and BGP-CN-zImage-with-initrd.elf.

Extracting files from an existing ramdisk image

To extract file from an existing ramdisk image, do the following (ION ramdisk only):

$ ./packages/tools/z-extract-cpio-from-ramdisk.sh  <existing_ramdisk_image> ramdisk.cpio
$ mkdir treeroot && cd treeroot
$ cpio -idv < ../ramdisk.cpio

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Kernel | Top | ZOID