[Borgbackup] Edge cases in disk image example in documentation

[Tomasz Melcer]

Hello,

I started looking at the suggested use of borg to back up disk images, [1].

  [1] 
https://borgbackup.readthedocs.io/en/stable/deployment/image-backup.html

I've noticed that the scripts shown there may have problems performing 
backups correctly in some edge cases.

Firstly, the suggested scripts don't deal with extended partitions. Only 
the main partition table is backed up. This may result in loss of data 
or difficulties restoring the backups.

Secondly, the size of the header is defined as everything before the 
"first" partition. However, the script considers the first partition as 
the one listed as first in the partition table, and not the one that is 
starting from the lowest sector. Curiously, the first partition in the 
partition table doesn't have to be the one being "physically" the first 
one on the disk. If this (rare) condition happens, the 
`hostname-partinfo` archive will be needlessly big.

The way suggested by the `sfdisk` man page to deal with these problems 
is to use its `--backup` switch — a bit bothersome to work with, but it 
seems to work for me.

I have a script that performs a similar procedure and which has been 
tested in these two corner cases. However, instead of storing partitions 
as separate archives, it firstly builds a "clean" version of the disk 
image, with zeroed-out sectors not used by filesystems or sectors 
between partitions, and only then stores the clean image as an archive. 
I found this procedure to be a bit more useful for my use case — I 
noticed I can start a virtual machine and choose a disk image straight 
from `borg mount`, which is pretty amazing!

I've posted my script at [2]. As it is different, it wouldn't outright 
replace the scripts from the page linked above. If you think it's proper 
to add it to the documentation next to the existing script, I'm willing 
to submit a patch.

  [2] https://gist.github.com/liori/ceac2a0c2fc7de1775a2585c4209f51b


-- 
Tomasz Melcer