Moving a slice

[james.cook at utoronto.ca]

On Sun, Mar 06, 2005 at 03:03:19PM -0300, Alejandro Pulver wrote:
> Hello,
> 
> I have two IDE hard disks, the first has W2K and WXP, the second has a
> GNU/Linux Debian Sarge (for booting purposes only) and a FreeBSD 5.3.
> 
> The Linux slice is the number 1, but the FreeBSD slice is number 4.
> There is a long story behind this, when I was looking for UNIX like
> Operating Systems (I had two Linux and one Linux Swap slices, but I
> removed them). At the end I choose FreeBSD.
> 
> Here is my slice layout:
> 
> # fdisk -s
> /dev/ad2: 79656 cyl 16 hd 63 sec
> Part        Start        Size Type Flags
>    1:          63    20466747 0x83 0x00
>    4:    40949685    38909430 0xa5 0x80
> 
> # fdisk
> ******* Working on device /dev/ad2 *******
> parameters extracted from in-core disklabel are:
> cylinders=79656 heads=16 sectors/track=63 (1008 blks/cyl)
> 
> Figures below won't work with BIOS for partitions not in cyl 1
> parameters to be used for BIOS calculations are:
> cylinders=79656 heads=16 sectors/track=63 (1008 blks/cyl)
> 
> Media sector size is 512
> Warning: BIOS sector numbering starts with sector 1
> Information from DOS bootblock is:
> The data for partition 1 is:
> sysid 131 (0x83),(Linux native)
>     start 63, size 20466747 (9993 Meg), flag 0
> 	beg: cyl 0/ head 1/ sector 1;
> 	end: cyl 1023/ head 254/ sector 63
> The data for partition 2 is:
> <UNUSED>
> The data for partition 3 is:
> <UNUSED>
> The data for partition 4 is:
> sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
>     start 40949685, size 38909430 (18998 Meg), flag 80 (active)
> 	beg: cyl 1023/ head 0/ sector 1;
> 	end: cyl 1023/ head 254/ sector 63
> 
> There is a lot of free space after and before FreeBSD slice.
> 
> I would like to know if is possible to move the entire FreeBSD slice
> (with 'dd', for example) to the end of the Linux slice, and then
> change the starting point in the slice table, and then change '4' into
> '2'. So there is no free space between the slices and the numbering is
> correct.
> 
> I think 'dd' will not overrite some parts of the slice with others
> because it is copying the data backwards, not forward.
> 
> Just for curiousity: Can I make 'dd' copy the data backwards (I mean,
> the same result but instead of copying 1 to <dst>, 2 to <dst>+1,
> etc.; copies <last> to <dst>, <last>-1 to <dst>+1, etc.)? Is possible to
> make such modification to 'dd'? I plan to use this to move a slice
> forward.
> 
> I guess I will have to use a bootable CD to boot a FreeBSD system (like
> FreeSBIE) to move the slice.
> 
> Here are my results in bytes to pass to 'dd' (are they correct?):
> 
> Part	Start			Size
> 1:	32256			10478974464 (9993 Meg)
> 4:	20966238720 (19994 Meg)	19921628160 (18998 Meg)
> 
> What does the line "Warning: BIOS sector numbering starts with sector 1"
> mean? It is an error?
> 
> Does the filesystem has to do with the phisical location of the slice
> (according to the BSD label I think is does not, because it uses
> offsets, not absolute values)?
> 
> Do I have to modify other file than '/etc/fstab' (like a loader
> configuration file)?
> 
> I appreciate any recommendations/considerations/instructions/warnings.
> 
> Thanks and Best Regards,
> Ale

I moved a FreeBSD slice from the end of my hard disk to somewhere close to
the beginning a month or so ago, using nothing but dd, and it worked.

Please note that the original space occupied by the slice and the place I
moved it to did not overlap at all, so I have no idea if you'll run into
problems when you overwrite the start of it.  It would probably work, but if
you have to start over for some reason, you're sunk -- and you'd need some
sort of boot media to get it to work..  Maybe you can copy it one partition at
a time?

Also, beware that the FreeBSD disklabel seems to use absolute offsets instead
of relative offsets.  If you copy the whole slice at once, something like

# bsdlabel /dev/{old slice} > /tmp/text-label
# bsdlabel -R /dev/{new slice} /tmp/text-label

should work, since the human-readable output uses relative offsets.  (But
beware; I'm just working from memory here and haven't tested those commands at
all.)

If you copy the data one partition at a time (as I ended up doing for some
reason) it's a bit more complicated: you have to set up the disklabel before
you start, but then when you copy partition a, the label will be messed up
(assuming partition a starts at the beginning of the slice).  I just fixed the
label again after copying partition a, and it seemed to work, but I can't
guarantee that the system won't re-read the label while dd is working and
decide that the destination partition is suddenly somewhere else and kill your
original slice.

Er, just sharing my experience.  You might want to wait for furthur guidance,
since all this is pretty messy.

- James Cook
  james.cook at utoronto.ca