Re: Multiple identical backups with different names

[Colin Percival <cperciva@tarsnap.com>]

On 12/20/15 06:45, Philipp Riegger wrote:
> Currently I have a backup script which creates backups with -daily-,
> -weekly- and -monthly- in the name for every day, week or month
> respectively. At the moment, those are completely different backups
> created at different times from the same data.
> 
> Is it possible to create 2 backups with different names from the same
> data at the same point in time without using snapshots or something like
> this? From my point of view it should be very easy to implement this in
> tarsnap (allow "tarsnap -c -f name1 -f name2 ...").

Not right now.  I don't think it would be impossible to add this, but it
would add quite a bit of complexity to the client code and if your system
died at the wrong time you could still end up with the archive existing
with one name but not the others.

> At the moment I can only think of something tarsnap-external like
> snapshots (which would be more difficult to implement for me) or
> something like "tarsnap -c -f name2 @@name1", but I think this would be
> quite expensive, is that correct?

As Bob said, copying archives is cheap; tarsnap needs to download the list
of blocks so that it knows they're still needed after the first archive is
deleted, but none of the actual data gets downloaded.  (You can do the same
thing with `tarsnap -r -f name1 | tarsnap -c -f name2 @-`, but that *does*
download the entire archive.  The @@-handling path does magic.)

Mind you, the cheapest way to create two identical archives is simply to
create two identical archives -- that way you won't download anything at
all.  But of course that would mean doing more work in the tarsnap client
to re-read and re-deduplicate everything.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid