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Re: Problem with NON-STANDARD install

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Le 01/08/2016 à 21:15, Joe a écrit :
>
> There are two chunks of storage for grub:

Actually three, or even four.

> the files, stored in a normal filesystem and available under /boot,

Actually /boot/grub. GRUB does not store anything directly in /boot.

> and the bootloader itself, which
> generally lives in the drive MBR, but can also live in a partition. The
> bootloader is machine code which is stored outside any filesystem.

The files in /boot/grub are also part of the bootloader. GRUB cannot 
display its menu or boot any OS without them.

The first chunk is the "boot image" and must be stored into a MBR (whole 
disk boot sector) or PBR (partition boot sector). As it must fit into a 
512-sector size, all it does is load the next chuck using a hardcoded 
block list.

If the boot image is in a PBR, the BIOS won't load it directly so it 
must be chainloaded by another (primary) bootloader. The primary 
bootloader can be a standard MBR boot code (MSDOS style) which just 
loads and boot the PBR of the primary partition which has the "boot" 
flag set, or another bootloader such as LILO or another GRUB instance 
installed in the MBR.

The second chunk is the "core image". For historical technical reasons 
its size is about 30 kB, sometimes a bit more. It has three possible 
locations :
- on an MSDOS-partitionned disk, in the unallocated space between the 
MBR and the first partition (MBR gap). This space used to be 31 KiB when 
partitions were aligned on cylinder boundaries, but is now ~1 MB since 
partitions are aligned on 1 MiB boundaries ;
- on a GPT-partitionned disk, in a dedicated unformatted "BIOS boot" 
partition ;
- or as a regular file in a filesystem appearing as 
/boot/grub/i386-pc/core.img (or /boot/grub/core.img with older versions 
of GRUB such as the one in Wheezy). This option is not recommended as 
the filesystem may move the blocks around and the location of the core 
image is hardcoded into the boot image. Also this option is not possible 
when /boot/grub is not on the same disk as the boot image or not on a 
plain partition, e.g. on LVM, some software RAID levels, encrypted volume...

The first two locations (not in /boot/grub) are called "embedded".

Now, the confusion comes when you select the device to install GRUB. 
Actually this only selects the location of the boot image, i.e. in an 
MBR (if selecting a whole disk /dev/sdX) or a PBR (when selecting a 
partition /dev/sdXN). The location of the core image will be chosen 
automatically : embedded if possible, as a file otherwise.

Embedding is not possible if the core image is larger than the MBR gap 
on an MSDOS disk, if there is no BIOS boot partition on a GPT disk, or 
if the boot image is stored in a PBR.

In any case, AFAIK the boot image and the core image must be on the same 
disk.

The third chunk is the files in /boot/grub. In order to be able to read 
them, the core image will include the required modules to read the 
partition table and filesystem and optionally software RAID, LVM if 
needed. Not sure about encryption. I never managed to boot with 
/boot/grub on an encrypted volume.

The fourth chunk is the kernel and initrd/initramfs files that GRUB 
loads to boot an OS, as they are part of the boot process (but not part 
of GRUB itself). It can be located anywhere GRUB with all its modules 
can read. It is usually in /boot.

If the boot image fails to load the core image, you'll get an error and 
can do nothing but reboot. This happens when the core image was not 
embedded and either filesystem blocks were moved around or the partition 
containing /boot/grub was deleted.

If the core image fails to find the modules in /boot/grub/i386-pc/, it 
will drop into a rescue shell with limited capabilities, allowing to 
display known devices and correct the $prefix variable. This typically 
happens after the partition which contained /boot/grub has been deleted.

If it fails to find the /boot/grub/grub.cfg file, it will drop into a 
full-featured shell (the same you start by pressing 'c' in the boot 
menu) instead of displaying the boot menu.

So, what did you mean exactly by "install GRUB on its own partition" ?

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Thread

Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-01 17:40 +0200
  Re: Problem with NON-STANDARD install David Wright <deblis@lionunicorn.co.uk> - 2016-08-01 19:10 +0200
    Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-01 20:10 +0200
      Re: Problem with NON-STANDARD install David Wright <deblis@lionunicorn.co.uk> - 2016-08-01 21:00 +0200
      Re: Problem with NON-STANDARD install Joe <joe@jretrading.com> - 2016-08-01 21:20 +0200
        Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-01 22:30 +0200
        Re: Problem with NON-STANDARD install Felix Miata <mrmazda@earthlink.net> - 2016-08-02 03:10 +0200
        Re: Problem with NON-STANDARD install Pascal Hambourg <pascal@plouf.fr.eu.org> - 2016-08-06 12:10 +0200
        Re: Problem with NON-STANDARD install Pascal Hambourg <pascal@plouf.fr.eu.org> - 2016-08-06 12:10 +0200
          Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-06 15:10 +0200
          Re: Problem with NON-STANDARD install Felix Miata <mrmazda@earthlink.net> - 2016-08-06 16:20 +0200
            Re: Problem with NON-STANDARD install Pascal Hambourg <pascal@plouf.fr.eu.org> - 2016-08-07 00:00 +0200
              Re: Problem with NON-STANDARD install Felix Miata <mrmazda@earthlink.net> - 2016-08-07 00:20 +0200
            Re: Problem with NON-STANDARD install Pascal Hambourg <pascal@plouf.fr.eu.org> - 2016-08-07 11:50 +0200
  Re: Problem with NON-STANDARD install Brian Flaherty <bxf4@yahoo.com> - 2016-08-01 19:30 +0200
    Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-01 20:30 +0200
  Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-05 20:50 +0200
    Re: Problem with NON-STANDARD install David Wright <deblis@lionunicorn.co.uk> - 2016-08-05 22:10 +0200
      Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-05 23:10 +0200
      [PRIVATE REPLY} Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-08-05 23:20 +0200
    Re: Problem with NON-STANDARD install Richard Owlett <rowlett@cloud85.net> - 2016-09-02 15:40 +0200
      Re: Problem with NON-STANDARD install Felix Miata <mrmazda@earthlink.net> - 2016-09-03 06:30 +0200
        Re: Problem with NON-STANDARD install Lisi Reisz <lisi.reisz@gmail.com> - 2016-09-03 12:40 +0200
          Re: Problem with NON-STANDARD install Gene Heskett <gheskett@shentel.net> - 2016-09-03 13:00 +0200

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