Re: [PLUG] Avoid Arvixe at all costs!

[Rich Kulawiec <rsk@gsp.org>]

On Thu, Mar 16, 2017 at 04:52:01PM -0400, Ronaldo Nascimento wrote:
> Genuine question, what is the difference?

[ between backups and copies ]

Let me explain by example.

Let's suppose that /home is your working filesystem and /home2 is its copy.

You create the file "/home/fred" today at 5 PM.  Tonight at 12:05 AM it gets
rsync'd to /home2/fred -- along with everything else in /home.  You now
have a copy of /home/fred in /home2/fred.

Tomorrow, you modify fred.  You now have the current version of fred
(Friday) in /home/fred, plus you have a backup of fred (Thursday in
/home/fred2.

Tomorrow night at 12:05 AM rsync runs again, and you now have the current
version of fred (Friday) in /home/fred plus a copy in /home/fred2.

What you no longer have -- anywhere -- is the Thursday version of fred.
That's because it was copied over (well, rsync'd over).  It's gone.

Contrast with this:

Let's suppose that /home is your working filesystem and /dumps is where
you keep backups.

You create the file "/home/fred" today at 5 PM.  Tonight at 12:05 AM it gets
dumped (via dump (8)) to /dumps/dump.thursday.

Tomorrow, you modify fred.  You now have the current version of fred
(Friday) in /home/fred, plus you have a backup of fred (Thursday) in
/dumps/dump.thursday.

Tomorrow night at 12:05 AM dump(8) runs again, and you now have the current
version of fred (Friday) plus a backup of fred (Friday) in /dumps/dump.friday
plus a backup of fred (Thursday) in /dumps/dump.thursday.

(Let's assume that you're using dump(8) in incremental mode with
monotonically increasing levels, so that only the files modified
since the previous day's run are backed up.)

The point of this comparison is that if you wanted a copy, the first case
gave you that; but if you wanted a backup, it didn't.  Not really.
But if you wanted a backup, the second case gave you that; but not a
copy.  Not exactly.  (Dump's output is a single file that contains
all of the files it backed up -- similar to tar, but quite a different
format and with different metadata.)


The larger point is that rsync makes copies, dump makes backups (and tar
makes archives, hence its name).  Each tool has its advantages
and disadvantages, and sometimes one can be substituted for another
is certain preconditions are met -- or if the task is some hybrid
of copies/backups/archives.  Folks have also built systems on top
of them (notably using rsync in recent years) that provide the sort
of incremental backup capability that dump's had since forever.

The trick is figuring out which problem you're actually trying to solve,
and having done that, selecting the right tool and using it the right way.

---rsk
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