Making a bootable backup (hard)disk... how?
wblock at wonkity.com
Sun Jun 10 12:23:15 UTC 2012
On Sun, 10 Jun 2012, Ronald F. Guilmette wrote:
> (I got the Wrong Impression, I think, because I have read assertions like
> "...dump backs up at the filesystem block level...". What does that mean
> exactly? Use of the term "block level" in this context makes me think of
> something operating along the lines of dd.)
Rather than reading just the contents of files, dump operates at a lower
level, backing up all of the blocks used by a filesystem.
> 1) In your example under the heading "Copying Filesystems", the second
> shell command line shown is:
> dump -C16 -b64 -0uanL -h0 -f - /usr | (cd /mnt && restore -ruf -)
> Is that correct? Shouldn't it actually be this instead?
> dump -C16 -b64 -0uanL -h0 -f - /usr | (cd /mnt/usr && restore -ruf -)
> I mean if the goal is to create a whole backup under /mnt that looks just
> like the whole original system, then shouldn't ``restoration'' of the /usr
> part of the original system go into /mnt/usr?
The mount command above that is
mount /dev/da0s1f /mnt
The "f" indicates it's a split filesystem layout, where /, /var, /tmp,
and /usr are separate filesystems in different partitions. That's a
traditional layout in FreeBSD. bsdinstall in FreeBSD 9 creates a single
filesystem, / . Restoring a dump of a /usr filesystem would go to
/mnt/usr in that case. (That would mean you are converting from the
split layout to a single filesystem, because with a combined filesystem
you can't easily back up just /usr with dump. dump does not "cross
filesystems" when making a backup.)
> 2) Towards the end of your document you mention rsync. Assuming that I
> have made a backup of my entire /usr partition (using dump&restore) into
> /mnt/usr and that at midnight every night from now on I want to simply
> refresh that and bring it up to date with the current contents of my
> actual /usr partition, what is the most proper way to do this? Should
> I use rsync for that? Or should I use dump&restore again? If that latter,
> then how exactly does that work? I mean if I do the pipeline from dump
> to restore as you have shown in your examples in your "Copying Filesystems"
> section, then what must I do in order prevent dump from dumping files that
> haven't changed? (And likewise, how do I prevent restore from trying to
> restore files under /mnt/usr that have not changed? Or is that answer to
> that question that I simply have to do the first thing, i.e. force dump
> not to dump any of the unchanged files?)
See the dump man page about dump levels. I have not tried a dump level
higher than zero with a dump|restore copy, but expect it to work.
rsync will do it, and would be my choice for that; start with -aH for
options, and --exclude might be used for some directories that don't
need to be copied.
Also see the sysutils/rsnapshot port for an interesting snapshot use.
Combined with a bootable FreeBSD like mfsBSD (http://mfsbsd.vx.sk/),
that might be a workable alternative to creating a bootable backup.
> 3) Assuming that I have a second disk to which I plan (now) to use
> dump&restore to make periodic copies of all of my ``normal'' (non-backup)
> filesystems onto. Assuming also that I've already installed the FreeBSD
> boot loader into the boot block on this second disk, and that I've already
> run fdisk & bsdlabel on it so as to set up all of the partitioning to be
> essentially identical to my ``main'' system disk. Assuming that all of
> this is the case, by using dump&restore as you have shown in your document
> under the heading "Copying Filesystems", will I have succeded at doing what
> I was first asking about in this thread, i.e. will I have successfully
> created a _bootable_ mirror of my main system disk... something that I
> can just plug in and go with, when and if disaster strikes and my main
> system disk suffers a horrifying head crash, you know, the day after the
> manufacturer's warranty period is up?
With some tuning, yes. /etc/fstab mountpoints often won't match when
such a drive is connected to a different port or new system. Use
gpart(8) GPT labels or tunefs(8) filesystem labels. Likewise with the
Ethernet board, so see rc.conf(5) about ifconfig_DEFAULT.
gpart and GPT are easier than fdisk and bsdlabel. I really need to flip
that disk setup article upside down, with gpart first as the new
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