BDR - Boot Drive Emulation
==========================

***

Note: this software is still in an early stage. Although there are currently
no open bugs, it is far from being well tested.

So, be careful.

***


Overview
--------

BDR is a small program loaded at system startup, typically from your hard
disk MBR, that makes a normal file available as disk drive in the BIOS.

So far so good. But what's the point?

The idea is that you can install your favorite bootloader (e.g. grub) into
an image file and use that instead of a separate boot partition.

Agreed, it sounds a bit weird at first; but there are several advantages
because BDR takes care of:

- device mapping

  You don't need to worry about a correct 'device.map' file for grub. BDR
  will sort out the drive mapping at boot time. That means you can rearrange
  your disks or put it into a new machine and the emulated boot drive still
  works (and you don't have to adjust your bootloader setup).

- 64 bit sector numbers

  The classic partitioning scheme works only up to 2TB (resp. 4TB, but only
  with tricks) due to 32 bit sector numbers. BDR works with 64 bit sector
  numbers and you can continue using your bootloader with large disks even
  if it only supports 32 bit sector numbers. (Your emulated drive will
  typically be substantially smaller than 2TB.)

- works with GPT

  If you partition with GPT (e.g. because the disk is > 2TB) you can still
  use a non-GPT-aware bootloader.

- resizable

  You can resize your boot disk any time you want.

- no separate boot partition

  If you need a separate boot partition, this may be a way to get rid of it.

- independent of filesystem

  As the image file can be on any (noncompressing) filesystem, you're not
  dependent on the support of your favorite filesystem in the bootloader. 
  (E.g. grub needs not follow the latest ext3/4 developments or you can boot
  with syslinux but still use jfs for your system partition.)

- RAID/LVM

  [* note: not yet *]

  Works with raid setups, even if your bootloader does not.


How it works
------------

BDR works in two steps:

1) Create a mapping.

  You create an image file, complete with partition table and a partition
  with a filesystem. Then run 'bdr' to map the file and store the metadata in
  a file 'bdr.map' on the first partition.

  You can set the drive geometry, BIOS drive id, and some other flags (e.g.
  make the drive read-only).

2) Attach the mapped file to your boot disk.

  Once step 1 is done, you install a small bootloader stub into the MBR of
  your boot disk (or all disks, if you're not sure which one is the boot
  disk) and point it to the image file.

  All this small boot program does is to assemble the new boot drive at
  startup and hook a driver for it to the BIOS. It then continues booting
  from the new drive. This means the image file must have a workable
  bootloader setup. See the example below how to do it for grub.


Setup example
-------------

***

A fancy wrapper script is needed. Here is how to do it step by step.

***


1) create a disk image file (e.g. 50MB):

# dd if=/dev/zero bs=1M count=50 of=/boot.img
50+0 records in
50+0 records out
52428800 bytes (52 MB) copied, 0.118283 s, 443 MB/s


2) partition it and add a filesystem (e.g. ext2):

# parted /boot.img mklabel msdos mkpartfs p ext2 0 100% unit s p
Model:  (file)
Disk /boot.img: 102400s
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start  End      Size     Type     File system  Flags                        
 1      32s    102399s  102368s  primary  ext2         , , , , , , , , , type=83, , 

*** Note:
 The start offset is 32 sectors, in bytes: 32*512 = 16384.


3) map the image file:

# bdr --create-map --bios 0x80 --geo 4,32 /boot.img 
mapping info created

*** Note1:
  4 heads, 32 sectors/track is the typical parted default geometry.

*** Note2:
  0x80 is the first BIOS disk. You can use other values (e.g. 0x83) but remember
  to adjust your grub setup accordingly (e.g. hd0 -> hd3).

You can look at the result:

# bdr --verify-map /boot.img 
/boot.img: mapping found
        id: c078d2fa72bc387e
      date: 2/4/2009
bios drive: 0x80
 disk size: 102400 sectors
  disk geo: chs 800/4/32
     flags: +edd, rw
     drive: 0 = /dev/sda
    sector: 2803723


4) now mount the image:

# mount -oloop,offset=$((32*512)) /boot.img /mnt

*** Note:
  We need the offset values from step 2.

You should see just the mapping file:

# ls -l /mnt
total 15
-rw-r--r-- 1 root root  3072 2009-04-02 14:30 bdr.map
drwxr-xr-x 2 root root 12288 2009-04-02 14:29 lost+found


5) copy your boot files to it:

# cp -a /boot/* /mnt


6) to simplify things, remount it to /boot:

# umount /mnt
# mount -oloop,offset=$((32*512)) /boot.img /boot


7) write a new device map

# echo "(hd0) /boot.img" >/boot/grub/device.map

Also, ensure you reference hd0 in /boot/grub/menu.lst. If you have several
disks in your machine, you might need to edit menu.lst a bit.

*** Note:
  See Note2 from step 3.


8) setup grub:

# echo "setup (hd0) (hd0,0)" | grub

If things went fine so far /boot.img is now ready to use as disk image. You can
check that the grub setup is ok using hexdump:

# hexdump -C /boot.img | head -30

shows a lot of numbers but you should also see the string 'GRUB' in the
ASCII dump on the right side.

Also,

# qemu -hda /boot.img

should show grub booting.


9) write BDR loader into MBR of your real boot disk:

# bdr --add-to-mbr /dev/sda /boot.img
/boot.img: mapping found
        id: c078d2fa72bc387e
      date: 2/4/2009
bios drive: 0x80
 disk size: 102400 sectors
  disk geo: chs 800/4/32
     flags: +edd, rw
     drive: 0 = /dev/sda
    sector: 2803723

/dev/sda: mbr updated

That's it. Next reboot should start grub from the image file.


10) fstab

To get the image file mounted at system startup add a line like

/boot.img /boot auto loop,offset=16384 0 0

to your /etc/fstab.