Re: grub nilfs2 module

[Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org>]

Hi, 

At Thu, 04 Feb 2010 16:54:56 +0900 (JST),
Ryusuke Konishi wrote:
> 
> Hi,
> On Thu, 04 Feb 2010 10:02:28 +0900 (JST), Ryusuke Konishi <ryusuke-sG5X7nlA6pw@public.gmane.org> wrote:
> > Hi,
> > On Wed, 03 Feb 2010 21:43:46 +0900, Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org> wrote:
> > > Hi,
> > > 
> > > I've written nilfs2 module for grub2 boot loader.
> > > 
> > > Here is the patch and mail I'v sent to grub-devel ML.
> > > http://lists.gnu.org/archive/html/grub-devel/2010-02/msg00025.html
> > 
> > Great!
> > 
> > The patch looks fine to me except some work, for instance,
> > rollforwarding and checksum based log validation is needed.
> > 
> > But meanwhile, this prototype seems enough to load kernels and
> > initrds.
> > 
> > I'd like to try it for my laptop.
> > 
> > > It may not be mature yet, but I was able to boot kernel
> > > from nilfs2 partition and mount it as rootfs.
> > > So you are able to boot the system that has only nilfs2 partition.
> 
> Let me ask you a question.
> 
> One thing which is important in NILFS is that every block in the
> partition can be moved by the garbage collection (except two super
> blocks).
> 
> So, we cannot assume that file blocks stay in a locked position even
> for unmodified files.
> 
> Doesn't this cause problems for the boot sequence of GRUB2 ?

That's a good question.

It's basically OK because there usually are room for additional code
besides MBR in PC disk format.  The room size is 32KiB followed by MBR.
The room is not required for linux, but historically for DOS compat.

See the link how modern disk layout is.
http://www.pixelbeat.org/docs/disk/

You can check it with the dd by following
# dd if=/dev/hda of=hda bs=512 count=3 skip=65
# hexdump -C hda > hda.hex
# dd if=/dev/hda1 of=hda1 bs=512 count=3
# hexdump -C hda1 > hda1.hex

NOTE, the differences between two dumps are, I think, mtime in a super block.

So if you install grub2 with root directory in nilfs2, 
grub-install will install nilfs2.mod into that additional 32KiB space
The space is out of nilfs2 patition, so GC won't move the blocks.

thanks

regards

> 
> Regards,
> Ryusuke Konishi
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> 


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Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org>
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