Mounting ISO image from USB at boot time

Is it possible to mount a ISO image from USB disk and to use it as a filesystem at boot time(with grub)? I ask it because I would like to put the kern开发者_JAVA百科el linux image and an ISO to be used as a filesystem(with fedora bootstrap) into an USB disk(without creating new partitions, etc.), as it is possible to do by using Qemu, for example.

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Qemu is a virtualization/emulation environment. Grub is a bootloader, designed to get a kernel loaded into memory and start it executing. Neither program is directly related to your question, although you could certainly use Qemu to execute a VM that uses Grub to start Linux to do what you want.

Modern Linux distributions create an initrd, which the bootloader puts into memory for the kernel to use as its initial root file system. The initrd does things like loading the modules necessary to access the hard disks where the real root file system lives. In your case, you should look at having the initrd find your ISO, mount it, and use it as the root.

The contents of initrd vary based on what distro you're using. I'd grab a livecd from somewhere, dump its initrd's contents with zcat /boot/initrd-2.6.whatever.img | cpio -id, and check out what it's doing. Look for the init file, which will be the first user-space process run by the kernel.

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Grub's loopback feature should allow you to boot a kernel and initrd from within an ISO image. Unfortunately, there's no way to allow the kernel to mount a loopback device as the root filesystem, so I think you're out of luck.