On Mon, Aug 4, 2014 at 5:15 AM, Gabriele Bartolini
<gabriele.bartolini@2ndquadrant.it> wrote:
> I really like the proposal of working on a block level incremental
> backup feature and the idea of considering LSN. However, I'd suggest
> to see block level as a second step and a goal to keep in mind while
> working on the first step. I believe that file-level incremental
> backup will bring a lot of benefits to our community and users anyway.
Thing is, I don't see how the LSN method is that much harder than an
on-disk bitmap. In-memory bitmap IMO is just a recipe for disaster.
Keeping a last-updated-LSN for each segment (or group of blocks) is
just as easy as keeping a bitmap, and far more flexible and robust.
The complexity and cost of safely keeping the map up-to-date is what's
in question here, but as was pointed before, there's no really safe
alternative. Nor modification times nor checksums (nor in-memory
bitmaps IMV) are really safe enough for backups, so you really want to
use something like the LSN. It's extra work, but opens up a world of
possibilities.