On Tue, Oct 21, 2014 at 04:40:48PM +0100, Laurie Brown wrote:
On 21/10/14 16:11, Chris Green wrote:
[SNIP]
I much prefer snapshots as they seem to me much safer and more robust.
So... Why ask about incrementals in the subject of the OP if you've already decided to use snapshots only?
Because, to my mind and many others, incremental can mean differential or snapshot! :-)
Er. No!
A snapshot is exactly that, it is a copy of a state at an exact moment in time, with no references to the past or future.
Incremental means - well, by increments. In the case of backups, it means that only those changes since the last backup are saved rather than the whole file. Each of those changes is by its nature incremental, so of course, it means rebuilding, using those increments, from the original outwards, in the correct order.
They simply are not the same. If you ask for incremental backups, that's what you'll get, not some kind of random definition of a snapshot, massaged to become a differential. Which is what exactly in that context? Seriously? Those are rhetorical questions, BTW. I realise they don't have an answer. Not sensible ones anyway.
Sigh...
Sigher! :-)
You do realise that snapshots use hard links to save space do you? Thus they do effectively have references to the past and future even though each is, as you say, a snapshot at the given time. A file that doesn't change has hard links from multiple snapshots to the same data on the disk so there's only actually one copy of it.
An incremental backup doesn't have to have increments of *files*, though it may do. The increments can be simply the increments of *file system* that are needed to build up the complete backup.
That's the problem in a way, the words don't really define things exactly enough.
Try doing a Google search for inremental or snapshot or differential backup and you will see they all tend to mean all things to all men.