Re: On markers of changed data

Alvaro, Michael,

* Alvaro Herrera (alvherre(at)alvh(dot)no-ip(dot)org) wrote:
> Michael Paquier wrote:
> > That’s actually what pg_rman is doing for what it calls incremental
> > backups (perhaps that would be differential backup in PG
> > terminology?), and the performance is bad as you can imagine. We could
> > have a dedicated LSN map to do such things with 4 bytes per page. I am
> > still not convinced that this much facility and the potential bug
> > risks are worth it though, Postgres already knows about differential
> > backups if you shape it as a delta of WAL segments. I think that, in
> > order to find a LSN map more convincing, we should find first other
> > use cases where it could become useful. Some use cases may pop up with
> > VACUUM, but I have not studied the question hard enough...
>
> The case I've discussed with barman developers is a large database
> (couple dozen of TBs should be enough) where a large fraction (say 95%)
> is read-only but there are many changes to the active part of the data,
> so that WAL is more massive than size of active data.

Yes, we've seen environments like that also.

Thanks!

Stephen