syslinux xfs support

[Chris Murphy]

On Oct 15, 2013, at 2:54 PM, Dave Chinner <david at fromorbit.com> wrote:

> On Tue, Oct 15, 2013 at 01:33:19PM -0700, Christoph Hellwig wrote:
>> Hi Paulo,
>> 
>> just wondering what the state of the xfs support for syslinux is?  I
>> talked to Peter at Linuxcon and he thought it's merged, but looking at
>> the kernel.org tree I can't find the support.
>> 
>> Also when looking over your branches I noticed that you're using a free
>> sector in the first filesystem block to store the bootloader.  If we
>> want to go down that route we need to make sure to reserve this sector,
>> otherwise it might get taken up by newly added metadata.
> 
> It's also worth pointing out that there's no guarantee that there's
> a free sector in the first filesystem block. It's only by luck that
> there's free sectors on the default config (512 byte sector, 4
> sector sized AG headers, 4k filesystem block). If we have <= 2k filesystem
> block there are no free "pad" sectors that can be used, 4k sectors
> mean no free sectors either, etc.
> 
> Much better would be to create a sector sized file and use fiemap to
> get the disk address of the block and feed that into the
> bootloader. That works for all filesystems without needing to know
> anything about the underlying filesystem structures……

I'm curious how that would work.

The minimum bit of code for GRUB or extlinux is a lot bigger than 4KB. A basic self-generated GRUB configuration file is 5.6KB; for extlinux a basic one I have is 518 bytes. The minimum code needed to find the configuration file is ~26KB for GRUB's core.img, and ~34KB for extlinux's ldlinux.sys.

So are both of you referring to the < 440 bytes of bootstrap code that goes in either the MBR or VBR, whose job is to find core.img or ldlinux.sys and blindly load them?


Chris Murphy