Ramdisk

Introduction

Both CN Linux kernel and ION Linux kernel requires ramdisk to boot. Ramdisk images basically contain minimum Linux utilities, init scripts , configuration files, kernel modules, etc, which are required by OS boot process.

ION ramdisk is an ELF file that contains a cpio format 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 Compute node
  • IBM CIOD is used instead of ZOID

Our ION ramdisks are similar to IBM default ION ramdisk but we add some extra files to support our features. The extra files are located in ramdisk/ION/ramdisk-add. The build-ramdisk script from IBM BGP driver is used to create ION ramdisk. The default path of the build-ramdisk script is /bgsys/drivers/ppcfloor. The build-ramdisk script path can be configured by the main configure script.

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

How to create ramdisk images

CN kernel image, ION kernel image and ION ramdisk images are always (re-)created from prebuild objects if you type make at the top level directory (without any make target),

In case you actually need to create ION ramdisk individually (without rebuilding other images), you can do:

$ make bgp-ion-ramdisk-cnl

If you want to create CN ramdisk(technically create CN kernel image with new ramdisk contents), type:

$ make bgp-cn-linux

NOTE: ramdisk/CN/bgp-cn-ramdisk.cpio.gz is a newly built CN ramdisk.

How to modify ramdisk contents

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

CN ramdisk

You can customize CN ramdisk by editing our CN ramdisk build script, which is ramdisk/CN/create-bgp-cn-linux-ramdisk.pl. You can add files with permission, delete files, creating device files, etc.

While we keep our CN ramdisk contents in ramdisk/CN/tree, you can add any file to the ramdisk as long as they are accessible from the script. You may be able to add login node's executables if they are a 32-bit PPC binaries and you copy all required files such as shared libraries, config files, etc.

Here is an useful example. Supposed that you need the od command in CN ramdisk. You can build the od command from source code from source code for CN ramdisk. If you want to do something quick, you can check see if login node's od command works or not.

$ 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 necessary shared libraries, so you simply can use login node's od command. What you need to do is to add one command to a perl array named @cmdlists in ramdisk/CN/create-bgp-cn-linux-ramdisk.pl and type make to recreate the CN ramdisk.

$ vi ramdisk/CN/create-bgp-cn-linux-ramdisk.pl # 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 permission 0755, uid=0 and gid=0.

The line that you added is a gen_init_cpio command. You can also create a directory, device file, symbolick link, pipe file, socket file as well.

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 gen_init_cpio commands in @cmdlists is matter.They are interpreted and executed from top to bottom. You can't add a file to the directory that has not been created.

CN Linux startup script

The first thing that Linux kernel does after kernel successfully boots is to execute an executable called the init program. The init program is usually /sbin/init that is internally busybox. Then /sbin/init executes a startup script defined in /etc/inittab, which is /etc/init.d/rc.sysinit.

Our startup script is a very small shell script and do minimum setup, starts telnetd to allow users to login from ION and starts the Zoid control process which takes care of job control.

In case you need to start your software at CN boot time, you can add command invocations to ramdisk/CN/tree/etc/init.d/rc.sysinit.

ION ramdisk

Unlike CN ramdisk, the range of customization is limited on ION ramdisk. You have no permission control. You can't create device nodes, etc. Currently we build the ION ramdisk using IBM build-ramdisk script by specifying add-on tree which contains our extra files.

What you can do are basically:

• Add files
• Overwrite files that are in the ramdisk created by build-ramdisk

Once you add files under ramdisk/ION/ramdisk-add/, they will be automatically added to the ramdisk. Here is an example 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's default location is /bgsys/drivers/ppcfloor/build-ramdisk. 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.

How to extract files from existing ramdisk image

If you want to extract file from existing ramdisk image, do the following steps (ION ramdisk only).

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