Re: grub nilfs2 module

[Ryusuke Konishi <ryusuke-sG5X7nlA6pw@public.gmane.org>]

On Thu, 04 Feb 2010 17:18:04 +0900, Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org> wrote:
> 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

Thanks! That cleared up my doubts on GC.
The reference you cited is informative.

Regards,
Ryusuke Konishi
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