Re: why does one need an initrd if using LVM?

[Werner Puschitz <wp@xxxxxxxxxxxx>]

On Sat, 3 May 2003, Werner Puschitz wrote:

> 
> On Sat, 3 May 2003, Robert P. J. Day wrote:
> 
> >   so why is initrd.img necessary for LVs even when it has
> > no modules?
> 
> The initrd image is needed to run a few initial commands within the initrd
> image (filesystem). The system loads the "linuxrc" file from the initrd
> image. This file is a script which scans the disks for volume groups,

BTW, if the kernel has been compiled with the CONFIG_BLK_DEV_INITRD 
option, which is the case with Red Hat's kernel:

$ grep CONFIG_BLK_DEV_INITRD /boot/config-2.4.18-26.8.0
CONFIG_BLK_DEV_INITRD=y
$

then you can see that the kernel always executes "linuxrc":

/usr/src/linux-2.4/init/do_mounts.c:

#ifdef CONFIG_BLK_DEV_INITRD
static int do_linuxrc(void * shell)
{
        static char *argv[] = { "linuxrc", NULL, };
        extern char * envp_init[];

        sys_chdir("/root");
        sys_mount(".", "/", NULL, MS_MOVE, NULL);
        sys_chroot(".");

        mount_devfs_fs ();

        close(0);close(1);close(2);
        setsid();
        (void) open("/dev/console",O_RDWR,0);
        (void) dup(0);
        (void) dup(0);
        return execve(shell, argv, envp_init);
}

#endif

static void __init handle_initrd(void)
{
#ifdef CONFIG_BLK_DEV_INITRD
        int ram0 = kdev_t_to_nr(MKDEV(RAMDISK_MAJOR,0));
        int error;
        int i, pid;

        create_dev("/dev/root.old", ram0, NULL);
        mount_block_root("/dev/root.old", root_mountflags & ~MS_RDONLY);
        sys_mkdir("/old", 0700);
        sys_chdir("/old");

        pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
        if (pid > 0) {
                while (pid != wait(&i))
                        yield();
        }

        sys_mount("..", ".", NULL, MS_MOVE, NULL);
        sys_umount("/old/dev", 0);

...

Werner


> etc. and mounts the root filesystem. For example, on my system linuxrc
> runs at the end the following command:
> mount -o defaults --ro -t ext3 /dev/Volume00/LogVol01 /sysroot
> 
> You can find the script "linuxrc" in the root directory of the initrd
> image. You will also see that the linuxrc script uses the script
> interpreter nash(8). This is a very small interpreter which has some
> important built in commands like mount, umount, etc. See man nash(8).
> You can find the nash interpreter in the bin directory of the initrd
> image.
> 
> So basically, you need the initrd image for finding and mounting the root
> fileystem. Here is a printout of my linuxrc script:
> 
> #!/bin/nash
> echo "Loading lvm-mod module"
> insmod /lib/lvm-mod.o
> echo "Loading jbd module"
> insmod /lib/jbd.o
> echo "Loading ext3 module"
> insmod /lib/ext3.o
> echo Mounting /proc filesystem
> mount -t proc /proc /proc
> echo Creating block devices
> mkdevices /dev
> echo Scanning logical volumes
> vgscan
> echo Activating logical volumes
> vgchange -ay
> echo 0x0100 > /proc/sys/kernel/real-root-dev
> echo Mounting root filesystem
> mount -o defaults --ro -t ext3 /dev/Volume00/LogVol01 /sysroot
> pivot_root /sysroot /sysroot/initrd
> umount /initrd/proc
> 
> Werner
> 
> 
> 
> 
>