Re: [PATCH] Backup old boot sectors before installation

[Isaac Dupree]

Zhu Yi wrote:
> On Fri, 2009-12-25 at 05:21 +0800, Robert Millan wrote:
> > I think making a backup is a fine idea, but I'd rather avoid the "option
> > creep".  It doesn't hurt to simply dump the files in /boot/grub/.  If user
> > later discovers that valuable data was overwritten, she can figure out
> > how to call dd just as she can figure out the right parameters for
> > grub-install.
>
> OK. My only concern is the start position of grub core.img (as in disk
> sectors) is a variable (see find_usable_region_xxx). This ends up the
> backup file is not a simple dump. I used below format in my patch.
>
> <512 bytes MBR> + <U64 start position> + <sizeof(core.img) old sectors>
>
> Option 2 is to put the start position in a separated file.
>
> Option 3 is to use the information in boot.img (as I used in my patch as
> a verification method) to get the start position, i.e. `od -j92 -N8 -An
> -td8 $grubdir/boot.img`.
>
> Option 4 is to backup everything from MBR to the end of core.img
> (including the holes in the middle). This ends up with a bigger image.
>
> Option 1~3 requires the recover has the knowledge of the backup file
> format somehow, but a simple dd is enough for option 4. What do you
> think?

If the partitioning had been changed in the mean time, 4 would overwrite 
it with the older values. Similar problem for a weird setup with 
partitions in between (I dunno, might be possible with GPT or other 
weirdness).

I think option 2 or 3 is soundest.  Option 2 would have three separate 
files of information, (right?), and there must be a pretty 
straightforwards `dd` option to skip to the correct start position. 
Option 3 would omit the "start position" file... but that file is really 
cheap, maybe we'd better keep things simple for restorers and stick with 
three separate files.(Or, we could do something weird like putting 
start-position somewhere in the filename of the backup.)

-Isaac