Re: [RFD] Support for installing on XFS partition?

[Andrei Borzenkov]

15.11.2016 23:30, Daniel Kiper пишет:
> On Sun, Nov 13, 2016 at 12:16:20PM +0300, Andrei Borzenkov wrote:
>> Syslinux MBR supports booting off XFS partition by adjusting PBR offset
>> (see
>> http://repo.or.cz/syslinux.git/commit/4298786ca19e9121568ecd4cc8b79d276ccfd24a).
>> Is it worth to support? This can only work if Syslinux MBR is actually
>> installed, so it would give false sense success in all other cases. OTOH
>> this could enable chainloading off XFS by explicitly loading PBR from
>> this offset using - probably, any - bootloader.
> 
> Make sense for me. However, I was not able to find anything/doc/spec which
> says that 4th sector is used for PBR in XFS. Could you point such thing?
> I would like to have some more details about that.
> 

commit a126f17f663c438ef264a459fa130951dbac780d
Author: Paulo Alcantara <address@hidden>
Date:   Fri Jun 8 16:20:15 2012 -0300

    EXTLINUX: Initial XFS filesystem support

    XFS filesystems are divided into a number of equally sized chunks called
    Allocation Groups (AGs). Each AG can almost be thought of as an
    individual filesystem that maintains its own's space usage.

    Each AG has the following structure:
         - A super block describing the overall filesystem info
         - Free space management
         - Inode allocation and tracking

    So, due to historical reasons (SGI IRIX's design of disk layouts), the
    first sector in the primary AG on XFS filesystems contains the
    superblock, which is a problem with bootloaders that rely on BIOSes
(that
    load VBRs which are located in the first sector of the partition).

    This patch adds support to the EXTLINUX installer to be able to install
    Syslinux in an XFS partition, but with a limitation (temporarily) of
    relying on the filesystem block size being 4+ KiB, otherwise there won't
    be enough space to store the Syslinux boot sector in the blank area
around
    the 2048 to 4096 bytes of the primary AG/superblock.