Re: Filesystem snapshots

[Graham Percival <gperciva@tarsnap.com>]

On Sat, Apr 23, 2016 at 08:50:22AM +0100, John wrote:
> I'd have thought my problem was commonplace. The machine is a
> server. It has to remain available continuously and can be updated
> at any moment. It handles, for example, SQL updates and email, I
> can't just switch it to read-only for the duration of the backup.

Of course the server needs to stay active!

> When Graham writes "Shouldn't you be making the backup from a
> read-only copy of your filesystem", yes, definitely. That's why I
> don't understand how anyone can archive from a home directory on a
> desktop without, for instance, closing X and logging out the user.
> Nothing short of that is going to allow the home directory to be
> made read-only. Unless a snapshot is taken at some level, and the
> snapshot used for the backup.

Hmm.  I suspect there's two different ideas about snapshots and backups here.
I'm not an expert in this field (actually, I've never used snapshots at all!),
but it sounds like you might be doing this:

[best viewed in a fixed-width font]

filesystem RW -+--> filesystem RO ---> run tarsnap -+-> filesystem RW
               |--> snapshot RW --> normal usage ---^ (merge) -> delete

What I'm suggesting is this:

filesystem RW -+--> (RW, run SQL, emails, etc) ---> filesystem RW
               |--> snapshot RO --> run tarsnap --> delete snapshot


Essentially, you would not be making your *home directory* read-only.
Instead, you could create a read-only copy of your home directory (which
continues to be read-write), then you archive that *copy*.

Please let me know if I've misunderstood your current approach.  If my guess
was accurate, then I'll make a non-ascii version of the above diagrams and
slap it somewhere on the tarsnap website to help other people.  :)

Cheers,
- Graham