Step 2: boot livedvd-amd64-multilib-11.2.iso from hard disk (with grub2)

After a few days experience of the gentoo livedvd run from usb, I think the usb drive should be saved for some other important data; I decided to copy the iso and run it from hard disk; usually a hard disk has large enough space, people could just copy all files inside /image.squashfs to a local partition, like the way on gentoo-wiki, but this computer assigned to me has a SSD drive default, has only 120G, furthermore, it's already partitioned long time ago, I don't have more space to back up all data on it and don't want to risk re-partition it; so I'd still like the way of loop mount iso on the fly, for saving disk space;
http://www.gentoo-wiki.info/HOWTO_Lightning_fast_install_from_LiveCD

/dev/sda3       240277504   250068991     4895744   83  Linux
/dev/sda5       129341440   132855807     1757184   82  Linux swap / Solaris
/dev/sda6       132857856   152387583     9764864   83  Linux LABEL=UBUNTU
/dev/sda7       152389632   210980863    29295616   83  Linux LABEL=HOME
/dev/sda8       210982912   211372031      194560   83  Linux LABEL=DEBIAN

The ubuntu partition already has grub2 default installed and home partition has a lot of remaining space, so I mkdir /gentoo under /home, put livedvd-amd64-multilib-11.2.iso there, rename or hardlink a short name gentoo.iso there; extract kernel/initramfs from the iso and put into /boot of the ubuntu partition, create a new entry there and no need to install another grub;

Notice that grub2 (version 1.9*) has slightly different syntax with grub1 (version 0.9*):

1. disk partition order, counter from 1, same as Linux partitions;
2. better not to change (append entries) in grub.cfg directly, use /boot/grub/custom.cfg instead; because every time later ubuntu upgrade or install new kernels would call update-grub2 that would overwrite grub.cfg; you could change /etc/grub/40_custom (that support script smart calculation) or /boot/grub/custom.cfg on that ubuntu partition; here I use custom.cfg;

Gentoo-11 ~ # cat /mnt/sda6/boot/grub/custom.cfg
menuentry "Gentoo11-Live Linux-3.0.0 (on /dev/sda7 {gentoo/gentoo.iso})" {
	  linux (hd0,6)/boot/gentoo/gentoo aufs_mem=2G cdroot=/dev/sda7 cdroot_type=ext4 isoboot=gentoo/gentoo.iso vga=791 splash=silent,theme:livecd-10 console=tty1 quiet nodetect doload=ahci dox
	  initrd (hd0,7)/gentoo/gentoo-initramfs.gz
}

Compared to previous menuentry in syslinux.cfg, here I removed some unused parameters (like "root=/dev/ram0 init=/linuxrc" because that is the default, I think I should recommend the Gentoo-Ten team to remove that, but not sure if any other computer need it, at least my computer doesn't need it, it could boot, as granted; the only useful parameters here are:

1. aufs_mem=2G, it means how large of tmpfs layered on top of squashfs by aufs; default it's 420M tmpfs, enough if you don't modify/install too many applications in live mode;
   
2. cdroot=/dev/sda7, specify a parameter saves parsing during booting; specify cdroot_type the same way;
   
3. isoboot=gentoo/gentoo.iso, you need to specify where do you save that iso inside that partition; it's required;
   
4. nodetect doload=ahci dox; depends on if you know your hardware, if no idea, autodetect is best, but I know my hardware, specifying it could save booting time;
   

The default gentoo kernel/initramfs should work this way I thought originally, but it doesn't, I have to look at the init script and change it a little, here is the diff: http://paste.pocoo.org/show/460334/

the initramfs is just cpio (newc format) + gz; extract and reconstruct in this way;

$ mkdir -vp gentoo-initramfs
$ zcat gentoo.igz |(cd gentoo-initramfs; cpio -d -i)
$ emacs -nw gentoo-initramfs/...; modify /init and /etc/init.scripts
$ (cd gentoo-initramfs/; find |cpio -H newc -o) |gzip -v9 >gentoo-initramfs.gz

Or

$ (cd gentoo-initramfs/; find |cpio -H newc -o) |xz -v9 >gentoo-initramfs.xz
Gentoo-11 ~ # (cd /home/gentoo/; ls -lh gentoo-initramfs.*)
-rw-r--r-- 1 root root 9.8M Aug 16 02:25 gentoo-initramfs.gz
-rw-r--r-- 1 root root 7.5M Aug 15 17:50 gentoo-initramfs.xz

The latest xz (aka. lzma2) could save 20% over "gzip -v9";

Update: use "xz --check=crc32" instead, because some latest xz support "none,crc32,crc64,sha256" or more types of integrity check, default as crc64; the kernel xz dec is "xz embedded" that only support "none or crc32",  kernel may report XZ_OPTIONS_ERROR, you could use "xz -vl ..." to check it, read Documentation/xz.txt for more;

/usr/src/linux-3.0-gentoo-r1/lib/decompress_unxz.c:
-=--:%%--F1  decompress_unxz.c   92% L364   (C/l Abbrev)

        case XZ_OPTIONS_ERROR:
                error("Input was encoded with settings that are not "
                                "supported by this XZ decoder");
                break;

# (cd gentoo-initramfs/; find |cpio -H newc -o) |xz --check=crc32 -v9 >../gentoo-initramfs.xz