XFS and booting
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Hi,
Quoted 7 lines · H. Peter Anvin
I have been looking at adding XFS support to the syslinux bootloader suite, and discovered, to my dismay:
No, for root partition installations because the XFS superblock is written at block zero, where LILO would be installed. This is to maintain compatibility with the IRIX on-disk format, and will not be changed.
This FAQ entry could probably be worded a bit more diplomatically. It is both the IRIX format (11+ years) and the last 6+ years of use of XFS on Linux by many, many people (and many SGI / other companies paying customers) that prevents this kind of change at this stage.
Quoted 2 lines
This means that it's impossible to write a boot loader that actually plays by the x86 platform rules and still can boot from XFS. Other than
Using lilo with an xfs root via the boot=/dev/hda and root=/dev/hda1 method (i.e. non-root-partition MBR) has been working OK for me and others. Is that not playing by the rules? Is that an option for your setup? It's a third option to the two you listed anyway (grub vs /boot, I mean).
Quoted 6 lines
the GRUB option of spreading itself all over the disk in places it shouldn't be, like the MBR, thus breaking e.g. softraid and creating all kinds of unnecessary interoperability problems.
Anyway, since it looks like the damage of not offsetting the filesystem has already been done, I'm trying to figure out what, if anything, can
Or, worded another way, "the damage of designing a system having an MBR in the first sector of space that is then allocated to filesystems" - it's a shocker of a layering violation.
Quoted 9 lines
be done about it. A standard MBR will never be able to boot an XFS, but perhaps a slightly modified MBR can be made to do that, without introducing filesystem-instance-specific issues. In particular, if there is space anywhere in the superblock for a "boot sector pointer", *and* there is a way to safely write this pointer, then a slightly modified MBR could detect an XFS superblock and re-read a boot sector at that offset.
Is this something that could be done?
It could be done - thats not the sound of me volunteering though ;) - there is space available in most XFS filesystem geometries that could be reclaimed for this kind of thing.
For example, for a 512 byte sector filesystem with a 4K blocksize, we have unused sectors at bytes 2048->4096 (following the space used for the first 4 sectors - SB, AGI AGF, AGFL, but before the first fsblock). This is the default mkfs geometry, so by far most filesystems have the free space here that could be utilised by a boot loader. And there's plenty of space in the superblock for a field with the meaning "the boot sector lives here". Actually, it'd be pretty straightforward to do this in XFS - the trickier part is working out in mkfs all of the geometries which do not result in freespace available for a boot sector, but thats not really very hard either.
The important question is, would anyone use it, if such a feature was available? Eric/I/... well, anyone could probably hack up the XFS code in a weekend to allow someone to test this out, but I'd want to be very sure it wasn't going to be wasted effort...
cheers.
Signature · 1 line
Nathan