How to recover using a full backup?

[Volker Wysk]

Am Montag, 3. September 2018, 10:33:20 CEST schrieb Colin Watson:
> On Mon, Sep 03, 2018 at 12:31:00AM +0200, Volker Wysk wrote:
> > I'm making full backups of my system, on an external hard disc. It's a
> > combination of incremental and differential backup (accoding to the definitions
> > on wikipedia). So there isn't included everything in each backup, but only the
> > difference to the last time (or one of the last times, to be correct). This
> > works well.
> > 
> > The backup contains almost everything, excluding only things like /tmp,
> > /home/.../tmp, /dev, /proc and a few others.
> 
> This is the kind of backup strategy I prefer too: it allows both picking
> out individual files for spot-restores and restoring the entire system.
 
> One caution: for full-system backups, you need to make sure that your
> handling of various bits of file metadata is exactly faithful, and there
> are some gotchas there.  You should check these things, both that the
> backup preserves them, *and* that your restore strategy preserves them:

I'm using Dar, see:
http://dar.linux.free.fr/

>  * symlinks are preserved as symlinks rather than copying the contents
yes.

>  * hardlinks are preserved (i.e. if two files share the same inode, then
>    this is true in the backup as well)
yes.

>  * extended attributes are preserved
yes.

>  * modification times are preserved
probably.

>  * numeric user and group IDs are used throughout, rather than assuming
>    that user and group names can be looked up in matching passwd and
>    group databases which probably won't be available when you're doing a
>    full-system restore (I got this wrong once and it was a right pain to
>    fix everything up manually)
not sure.

>  * sparse files are handled efficiently (this isn't vital, but if you
>    have any sparse files on your system then you can end up with the
>    restored system taking up a lot more disk space than it used to
>    otherwise)
again, not sure.

I've asked Denis Corbin, the author of dar, about those requirements. (Waiting for answer) They are probably all met. Dar is not bad at all.


> You may want to make a backup of the disk's partition table structure,
> e.g. using sfdisk.  You can just leave this in a file that gets backed
> up along with the rest of the system.  Check that you have nothing in
> other partitions that you can't reconstruct.

What would one do with the disk's partition table structure..?

 
> > Then, I would need to boot from a different drive, such as an USB stick, make a
> > new file system on the (new or old) disc and restore the full backup in there.
> > 
> > I wonder how this is done. Just unpacking the backup doesn't do it. According
> > to what I know, there should some "update-initramfs -u" and "grub-install"
> > commands be necessary. But first some "mount --bind" commands are necessary,
> > such that the configuration files are in the right places. And I don't know if
> > there is more needing to be considered.
> 
> My usual strategy is to retain a copy of the UUID of the old file
> system, which doesn't involve any extra backup work; you'll have it
> somewhere in there, e.g. in /boot/grub/grub.cfg.  Then after restoring
> files I grep for that in /etc and update any matching files to refer to
> the UUID of the newly-created file system.  (Note that this is not
> necessary in the case where you're simply reverting to an earlier state
> after a catastrophic sysadmin failure, since in that case you don't need
> to create a new file system.)
> 
> After that, you've got it all - the procedure goes roughly like this:
> 
>   mount --rbind /dev /mnt/dev
>   mount --rbind /proc /mnt/proc
>   mount --rbind /sys /mnt/sys
>   chroot /mnt
>   update-initramfs -u
>   grub-install [possibly with suitable arguments]
>   update-grub

This should be "grub-install /dev/sda".

> Then unmount everything and reboot.  By definition you have recovery
> media at this point, so even if you do need to iterate a little bit it
> won't be too bad.

Thanks for your competent and elaborate answer.  :-)  Even if I don't fully understand yet.


Bye
Volker